Chapter 21 Chapter 11 The Cause of Evolution: Natural Selection-1

By the summer of 1837, Darwin was an unquestionable evolutionist.He has learned that species can change, that species reproduce by natural processes.But how such changes occur and what causes species change has always puzzled him.Fortunately for historians, Darwin kept all his thoughts and speculations in notebooks, and the rediscovery of these notebooks can re-trace the tortuous paths of Darwin's hypothetical reasoning.Darwin, like Lyell, had already considered the emergence of new species on the Beagle, when he was still a creationist, and so necessarily adopted the catastrophist model (e.g., the South American three-toed ostrich). origin of the second species).In these early musings Darwin encountered a pair of sympatric species occurring in the plains of Patagonia.

On this plain Darwin could neither find isolation nor, in the case of successive species, could he explain it by applying Lyell's theory of filling vacant habitats by new species.He was unable to find evidence of climate change so that earlier species could not go extinct.In the case of the giant llama, however, extinction did occur again, whose vacancy has now been filled by the sorrel vicuna (guanaco). Kohn (1981) and others articulate Darwin's thinking at this stage very clearly. In July 1937, Darwin began to take notes on the first (B) of his four notebooks marked B, C, D, and E (named "Notes on the Evolution of Species").The situation recorded in these four notebooks reflects the tortuous and complicated evolution process of Darwin's thought during the period of about 15 months after Darwin realized the theory of evolution by natural selection.Since it was a very complex theory, it naturally could not have been conceived in a flash, although Darwin could remember the exact date when he had an important inspiration.In Darwin's Autobiography (edited by Barlow) (1958:

120), he once compressed the slow and complicated development process of this theory into a moment and described it in a memorable passage: In October 1838 (actually September 28), that is, fifteen months after I started my systematic research, I happened to read Malthus's theory of population for entertainment. The struggle for existence, which is both going on, has been mentally accepted, and it occurs to me that in these cases favorable variations will be preserved and unfavorable variations will be eliminated.The result will be the formation of new species.From this I at last found a doctrine upon which to work.

What happened on September 28, 1838?It seems clear from his notes that it was the following sentence in Malthus's book that touched Darwin to a spring of wisdom.The sentence reads: "It is therefore safe to say that, if left unchecked, the population will double every twenty-five years, or grow geometrically." As we shall see below, the causal chain of the natural selection theory is quite logical.However, Darwin did not do it all at once, but first proposed a series of alternative theories, and then reviewed and discarded them one by one.But he was able to keep the useful part of the abandoned theory, and finally formed the theory of natural selection.Even the doctrine itself was not conceived in a day. Schweber (1977) attributes much of the evolution of Darwin's thought to his reading of Brewster and Quetelet in the two or three months before he came across Malthusian population theory. Kohn (1981) also seems to think that most of the theory was assembled by the end of September 1838 (however under different influences than that proposed by Schweber). Hodge (1981) also seems plausible that the decisive change in Darwin's thinking occurred in November 1838. Ospovat (1979), on the contrary, argued that Darwin's concept of natural selection and the nature of adaptation was far from mature in 1838 and would take several years to reach

The form stated in , that is, the form made public.The above-mentioned scholars agree on one point, that is, this theory was developed slowly step by step.Indeed, even in his later writings Darwin was often inconsistent when he spoke of choice, and sometimes contradicted statements he made at nearly the same time. In the three years following his return from the Beagle voyage Darwin read a vast literature, almost as much non-biological as biological (Herbert, 1974; 1977).It is obvious that Darwin did not live in a vacuum of knowledge and reason, but was always in close contact with the ideas that formed the zeitgeist of that era.This, not surprisingly, raises the question of to what extent Darwin's new ideas arose from his scientific discoveries, and to what extent he adopted or merely modified existing ideas among his contemporaries.In general, biologists tend to minimize external influences on this issue; non-biologists, intellectual historians, and social historians tend to the other extreme.

The name "Malthus" prompted one school of social historians to suggest that it was Malthus's theory of society that led Darwin to the theory of evolution by natural selection (see text).This statement is strongly opposed by historians of biology.However, as I have pointed out, historians of biology also have major differences in interpretation because of the complexity of Darwin's mode of explanation.The key ingredients of a new theory in the physical sciences are generally determined by a single factor, whether it's gravity, relativity, the discovery of electrons, and others.In contrast, biology, especially in the field of evolutionary biology, is highly complex.For example, Darwin's theory of evolution through natural selection has eight main components, some of which can be subdivided.More importantly, what usually plays a decisive role in a biological theory is how to explain the interactions between its main components.In order to determine exactly where Darwin was influenced by Malthus, it is necessary to dissect Darwin's mode of interpretation.The nature of his patterns can be pressed.origin of species.The titles of the first five chapters of the book are to build up the concepts in the mind; the titles of these five chapters are: "Variation Under Domestic Conditions", "Variation Under Natural Conditions", "The Struggle for Existence", "Natural Selection" and "The Laws of Variation ".

Darwin's theory of natural selection consists of three lines of reasoning based on five facts drawn partly from population ecology and partly from genetic phenomena. Fact 1: All species have such a strong potential reproductive capacity that if all born individuals reproduce successfully, the population (individuals) will grow exponentially (Malthus called it a geometric progression). Fact 2: Except for small annual fluctuations and occasional large fluctuations, populations are generally stable. Fact #3: Natural resources are limited.In a stable environment, natural resources remain relatively constant.

Reasoning 1: Since the number of individuals produced exceeds the carrying capacity of available resources, but the population size remains stable, it indicates that there must be fierce competition for survival among the individuals of the population, and the result is that in each Only a fraction, and often a very small fraction, of the descendants of a generation survive. The above facts from population ecology lead to important conclusions when combined with certain genetic facts. Fact Four: No two individuals are identical: The reality is that every population exhibits great variability.

Fact #5: A large fraction of this variation is heritable. Reasoning #2: Survival in the struggle for existence is not random or accidental, but depends in part on the genetic makeup of the surviving individuals.This non-uniform state of existence constitutes the process of natural selection. Reasoning 3: This process of natural selection will cause the population to change gradually over many generations, that is to say, lead to evolution, leading to the production of new species. The historian of science must ask which of these facts were new to Darwin, and if none were new, why didn't those before him reason the same way?He must also ask in what sequence did Darwin arrive at these foresights step by step?Why is the exponential population growth mentioned by Malthus so important to the final composition of Darwin's logical framework?

Before analyzing Darwin's theory in detail, it is necessary to make a brief introduction to Darwin's thought during the critical period of 1837-1838.Reading books made him realize the importance of gradualness in all changes.He was flatly opposed to sudden origin. "Natura non facit saltus" (Natura non facit saltus) was his motto, just as Lamarck once used it as his motto.This point of view is also completely consistent with Leyle's anti-disaster theory (see Chapter 8). Another point that must be noted is Darwin's original emphasis on diversity.Darwin always had his own point of view or theory about every state of affairs, and had his own set of theories or theories about the formation of island species long before he conceived the theory of natural selection.His theory of speciation is that if a group of animals becomes isolated from its host species population, it will gradually change under the influence of new environmental conditions until a different species forms.Concerning this doctrine Darwin once said "My theory is very different from Lamarck's" (Notebook B: 214), referring to his noting of Lamarck's "evolution by will".In fact his doctrine seems to be quite similar to the later neo-Lamarckian change due to local environment (Ruse, 1975a: 341), which is a purely model theory, where isolated populations of species react as a whole to new environmental conditions. same reaction.It is quite amusing that in Darwin's later years, after he had long since abandoned the doctrine, he blamed (grossly wrongly) Wagner for having a similar view and emphasized that "neither isolation nor time itself alters species" (L.L.D. II: 335–336).There are many examples that can be cited from Darwin's notes to illustrate his early theories, and I will mention only two here. "According to this view, the animals on separated sea islands, if separated long enough, should become different due to slightly different environmental conditions" (B: 7). "As I said before, 'isolated' species, especially 'isolated' species with some changes. Maybe become faster" (B:

17). There is no doubt that the years 1837-1838 were the most intellectually active period of Darwin's life.He greedily read all kinds of books and magazines, not only geology and biology, but also a lot of philosophy and "metaphysics".It was during this time that Darwin turned sharply to agnosticism, and his ideas of populations began to take shape, relying less on soft inheritance (Mayr, 1977a).Some of these facts are directly reflected in his notes, while others can only be inferred.This was a period of radical change of direction for Darwin, and it is no wonder that by 1838 many facts and concepts long known to Darwin had taken on an entirely new meaning. 11.1 The main components of the theory of natural selection Perhaps no concept in the history of thought has been more original, complex and bolder than Darwin's concept of explaining the mechanisms of adaptation. Many scholars have attempted to reconstruct the steps by which Darwin arrived at his final model.They try to fit a whole set of facts and opinions into a new framework.Instead of taking this somewhat chronological approach to analysis (for which I present only the relevant literature), I present some of the main concepts that make up Darwin's theory, and analyze their pre-Darwinian history and their place in thought. The prodigious capacity of living things to reproduce is a favorite theme of writers who describe nature.Only some scholars with whom Darwin was very familiar are mentioned here. Buffon, Erasmus Darwin, Paley, Humbert and Lyell have all talked about fecundity in their works.Darwin was particularly impressed by the astonishing rate of reproduction of protozoa, which he learned from C. G． Ehrenberg's work (Gruber, 1974).Darwin's failure to incorporate it earlier in his theory of evolution may have been mainly due to two factors.The first factor is that Darwin apparently did not yet understand that organisms with small numbers of offspring (such as birds and mammals) have the same potential for exponential growth as microbes.Another factor (discussed below) is that high fecundity is irrelevant in the essentialist frame of mind.If all individuals are identical, it does not matter what fraction (percentage) of them are eliminated before reproduction.The ability to reproduce became an important part of Darwin's teachings only when certain other points of view in Darwin's thought were fully developed. Social thinkers had been concerned with the question of human reproductive capacity many generations ago, and Malthus had little innovation in this matter.In fact he formed his view of population growth in geometric progression with direct reference to Franklin's calculations.Buffon and Linnaeus (Limoges, 1970) also proposed some calculations long ago to show how quickly a single species could fill the world if it multiplied without restriction.The natural theologian Paley (1802), one of Darwin's favorite authors, also said that "reproduction proceeds in geometrical progression . . . [while] food increases only in arithmetic progression." Did Darwin forget Had he read this in Purley's book? (It is also possible that Perley knew this from the first edition of Malthus's On Population, published in 1798). Half a century before Darwin, there was a dramatic change in how people interpreted the harmony of nature.Natural theologians resumed the study of the perfect harmony shown in the interaction between plants and animals and their environment, which had long been prevalent among some Greek philosophers.Everything is arranged in balance with everything else.If a species becomes slightly too vigorous, something happens to bring it back to its earlier level.The carefully orchestrated interdependence of the various forms of life is evidence of the benevolence and wisdom of the Creator (Derham, 1713). It is true that predators must hunt and kill prey, but predators must live once they are created by God.Prey are designed in this way in advance to provide overpopulation as food for predators.The apparent struggle for existence is only superficial; it never disturbs the basic harmony.The harmony of nature is extremely powerful, and species can neither be changed nor extinct, otherwise the harmony will be disturbed, and the species does not need to be improved, because there is no higher level of completeness. The concept and even the word "struggle for existence" is quite old, as Zirkle (1941) points out, was often cited in the seventeenth and eighteenth centuries.On the whole, however, this rivalry was seen by Linnaeus (Hofsten, 1958), Kant, Herder, Cuvier, and others as quite favorable for the necessary correction of the balance of nature.As knowledge of nature increased, a contrary interpretation arose, and the sharpness and brutality of the struggle for existence began to be perceived with increasing persuasion, and thus became widespread.Some works of Buffon and some remarks of Linnaeus have shown this point, as well as some articles of the German historian Herder; His views are also adopted in the long essay on the poignancy of the struggle for existence.Darwin got it from Leyle's, not Malthus's.It was the first time I came into contact with the concept of survival competition. When the fossil record shows how many species have disappeared, and when the research of geologists reveals how much the world has changed through geological time, the unchanging harmony of a designed world is of course untenable. of. Lamarck tried to save the concept of a benign balance in nature by negating extinction, that is, by using evolution to explain the disappearance of species.Once such an explanation is accepted, it leads to the shattering of the belief in a static world. Adaptation (as long as the concept exists) can no longer be seen as a static state, a product of the creative past, but as a continuous dynamic process.Living things are doomed to extinction unless they constantly change themselves to conform to the changing physical and biological environment.This change is ubiquitous, as climate changes, competitors invade, predators go extinct, food resources fluctuate, and virtually no component of the environment remains constant.When this was finally recognized, adaptation became a scientific problem. After 1837, Darwin's interest gradually shifted from the problem of diversity to the problem of adaptation. Darwin tried to analyze in more detail what factors caused the struggle for existence.Competition is of course a consequence of the three facts he listed above, that limited resources place limits on potential population growth.From the seventeenth century, and perhaps even earlier, some scholars have emphasized certain factors for stabilizing population numbers. In 1677, Matthew Hale listed five major limiting factors to population growth: epidemics, famine, war, flood, and fire.Linnaeus (Gruber, 1973) once made the following vivid description: "I don't know what kind of natural intervention or law keeps the human population within the appropriate range. However, most infectious diseases usually occur in the population It is true that dense areas erupt and rage, and I have often thought of wars where there is too much population. At least it seems to me that where the population increases too much, the harmony and the necessities of life decrease, which is true. Envy and hostility among neighbors increases. The result is a general war." Although Linnaeus has vividly described it, the struggle for existence rarely takes the form of a real confrontation (fighting).Generally nothing more than competition for a limited supply of resources.In the days of essentialism, competition usually referred (especially when applied to plants and animals) to competition between species.The decisive turning point in Darwin's thinking came when Malthus's treatise on fecundity came to fully appreciate the importance of competition among individuals of the same species How completely different is the style (type) of competition. In Darwin's time, philosophers of science (such as Herschel, Whewell) and statisticians (such as Quetelet) attached great importance to and emphasized quantification.Therefore, some scholars (such as Schweber, 1977) suggested that the reason why Malthus's argument touched Darwin so deeply is that this argument was expressed in quantitative language (geometric progression).It is indeed possible that this increased the appeal of Malthus's account to Darwin, although the "law of natural selection" is by no means a quantitative or predictive law.This is why Herschel later called natural selection the "law of higgledy-Piggledy".His definition just illustrates the philosopher's view of qualitative, indeterministic generalization. Some scholars have recently pointed out that in the first few decades of 1838, two concepts gradually changed, namely, the nature of the competition for existence (from mild to fierce) and the role of competition (from species to individual), but so far it has not A step-by-step thorough analysis is lacking.The existence of intraspecific competition had been perceived to some extent before Darwin, but did not seriously affect the concept of natural balance.Yet this is exactly what Malthus did to Darwin: "Even De Candol's fierce language does not convey the message of the struggle of species as it might be deduced from Malthus, who said that the escalation of violence must and It can be checked only by positive checks, except that famine may eliminate desire", (Note D: 134).Darwin was quite right in pointing out that until then it had been assumed that animals had as many offspring as they "needed".The idea that reproductive rates are mostly independent of gaps (gaps) in the organization of nature is incompatible with the natural theologians' concept of natural balance.The teleological view that members of a species can have as many offspring as they need has not been abandoned, and even in modern times David Lack (1954) has to contend with it. Darwin recounted in his autobiography and correspondence that he had long been aware of the importance of artificial selection, but did not realize how to apply this insight to evolution until he read Malthus.For example, he wrote to Wallace in 1858: "I have drawn from my studies in the breeding of domestic animals the conclusion that selection is the agent of change; apply this principle." Limoges and other modern scholars have questioned the evolution of this view, since Darwin never used the word "selection" in his notes until he saw Malthus's work but used "Pick" (Pick), and they don't understand why the study of livestock breeding can affect Darwin's views. But Wood (1973) and Ruse (1975a) point out that Darwin read extensively on animal breeding before his theory was formed. In the literature, when he read the pamphlet written by Sebright and Wilkinson in the spring of 1838, the underlining under the key sentence clearly shows that Darwin was very expert in artificial selection and attached great importance to it. Related to this, some of Darwin's friends who studied at Cambridge University were gentry sons, all keen on horseback riding and hunting, and undoubtedly had a certain interest in agriculture and animal breeding.Otherwise it is natural to ask how Darwin could have discovered so early on the importance of animal breeding to his work. Darwin got both good ideas and bad ones from breeders.It is not true, for example, that breeders are convinced that the variability of animals or plants can be increased simply by subjecting them to domestic conditions.Fortunately, Darwin also learned some very valuable concepts from the breeders, the most important of which was of course the emphasis on the individuality of each animal in a herd.It was this insight, rather than the actual manipulation of artificial selection, that led Darwin to realize a key part of his theory of natural selection. Years after this Malthusian episode, Darwin repeatedly spoke of how he arrived at the concept of natural selection by contrasting it with artificial selection.However, neither his notebooks nor the dated September 28, 1838, nor other parts of his notes attest to this recollection.While reading the animal breeding literature no doubt gave Darwin some important insights, there are also indications that he did not make this comparison until a few months later when it occurred to him that artificial selection is a brilliant experimental demonstration of natural selection. Hodge (1981) argues that this occurred in November 1838, when Darwin revisited his native Shropshire. Darwin's new model of natural selection was purely deductive, and following the advice of some of the leading philosophers of the time (Comte, Herschel, Whewed), Darwin felt obliged to present arguments for the soundness of his theory, ideally like the physical sciences It is proved by experiments as a convention of .But evolution from stem is so slow, how can we experiment with evolution?It was at this crucial point that Darwin was reminded of the activities of animal breeders.Darwin concluded that artificial selection was a microcosm of greatly accelerated natural selection, which provided the proof Darwin so desperately needed.Darwin paid special attention to artificial selection in his later years, and even believed that artificial selection provided the initial inspiration for his theory of natural selection, but the actual situation does not seem to be the case. Perhaps the most revolutionary change in Darwin's thinking in 1838 was the recognition of the specificity (uniqueness) of each individual. This specificity was of course something Darwin observed from everyday experience.Who doesn't know that no two people, or two dogs, or two horses are exactly alike?Every animal breeder takes for granted the individuality (characteristics) of each member of his herd.It is this that makes it possible for breeders to alter the characteristics of their herds by carefully selecting specific individuals to serve as sires and dams for the next generation.Yet precisely because it is so common, this personality has been largely ignored by philosophers.Once Darwin understood the importance of individual specificity, all his work over the next twenty years reinforced this new insight.His taxonomic work on barnacles is particularly convincing.Darwin found that individual variability was so marked that he was repeatedly confused whether two specimens were two different species or two variants of the same species.This variability is not limited to external morphology, but affects all internal organs as well.Whether the emphasis on the political individual ("individual rights") or the emphasis on certain schools of philosophy at the time also had an influence on Darwin's thinking is uncertain, and I find it dubious. It was the "discovery" of the importance of individuals that led Darwin to move from the concept of patterns to the thinking of populations.It was also from this that he realized that the struggle for existence caused by competition that Malthus so vividly described was a natural phenomenon involving individuals rather than species.By introducing the idea of ​​populations, Darwin caused a fundamental revolution in biological thinking.As introduced in the second chapter, this is a unique concept of biology, which is completely opposite to the thinking of physical scientists.Adopting the idea of ​​populations and denying the ideas of essentialists are closely related.As far as the essentialists are concerned, variation is unimportant and therefore of no interest to them.Changing characters in the vocabulary of essentialism are "merely accidental" because they do not reflect essence.It is also interesting to read the writings of Darwin's critics (Hull, 1973), for it appears that they were dumbfounded by Darwin's identification of variation as the most important feature of life.As far as the philosophical literature is concerned, this lesson has only been learned by a very small number of people. A new book by Toulmin (1972) is a notable exception.Those who still doubt the power of natural selection continue to use the essentialist arguments invariably.Darwin himself, too, turned gradually and slowly to population thinking, and in many of his speeches after 1838 still often reveal pattern language. 11.2 Origin of the Concept of Natural Selection Ask any biologist which concept is most typically associated with Darwin's name and he will answer: natural selection.This was an important new concept that Darwin introduced into biology, not just biology actually but an important concept that was introduced into everyone's mind.However, it has been repeatedly mentioned that this is not a completely new concept (principle) related to Darwin, but has been proposed frequently since ancient Greece (eg Zirkle, 1941).In order to affirm or deny the plausibility of this claim, it is important to distinguish between two processes that have been consistently confused in the literature.I call the first process the "elimination" process.There is a concept that there is a conservative force in nature, which eliminates all individuals that are not "normal", that is, all individuals that do not have the completeness of the common model.Such elimination fits perfectly with essentialism.For the essentialist, on the other hand, selection is obviously impossible, since the essence is unchanging and all variants are mere "accidents", such as the appearance of monsters (deformations) and other "desradations". In the 17th and 18th centuries the term "degeneration" was commonly used to describe biological change.If a significant degradation is survivable it constitutes a new "mode" (type).In fact the whole natural ladder was originally proposed in descending order of completeness (degeneration).However most degenerates are non-viable, they cannot survive or reproduce and are eliminated, thus maintaining the purity of the model.Weeding out obviously inferior or simply unfit individuals does happen all the time and is part of natural selection.This is called "stabilizing selection" in modern evolutionary biology (Schmalhausen, 1949; Waddington, 1957; Dobzhansky, 1970). Zirkle (1941) in his historical study lists many instances of "natural selection before publication" since the Greek Empedocles.Virtually all of these older instances he mentions refer to "elimination".Examples include Lucretius, Diderot, Rousseau, Maupetuo and Hume.In the case of Prichard, Spencer, and Naudin, model improvements are due to "Lamarckian" forces, such as use or disuse (use it or lose it), the exercise of ability, or the influence of the environment, while elimination is always Is to eliminate inferior patterns. Eiseley (1959) strongly argued that it was Edward Blyth who put forward the theory of evolution by natural selection in 1838, and believed that Darwin must have read Blyth's article and drew inspiration from it, but in his book But nothing is mentioned in the work.From Darwin's notes later discovered, Eisley's opinion can be refuted.More importantly, Bluff's theory is clearly a theory of elimination rather than a theory of competition.His main concern is maintaining the completeness of the schema.Bluff's thought was unquestionably that of the natural theologian, who held that variation "is one of the most prominent influences of design, an influence which clearly and forcefully demonstrates the omnipotent, great first cause existence.” Everything is a testament to the perfect balance of design and the natural world. (Schwat Z, 1974).Darwin may well have read Bluff's article but did not pay attention to it, since its thrust was against evolution, and its arguments were no different from those of other natural theologians.Bluff later became one of the scholars with whom Darwin relied heavily and frequently corresponded. There are two main reasons why the concept of natural selection was incompatible with Western ideas before the 19th century.The widespread essentialist thinking behind the first reason does not allow any idea of ​​gradual improvement (progress).All it allows is the sudden emergence of new models (types) and the elimination of inferior models.Articles by naturalists at that time only compared species when discussing adaptation phenomena, never referring to individuals.Another reason is the widespread acceptance of the equally prevailing teleology, which holds that the Creator's design automatically achieves completeness (perfection).It would be considered disrespectful (not to mention heresy) to seek mechanisms to improve this completeness.In natural theology there is simply no room for improvement of the model by natural selection. The process of natural selection as Darwin understood it is fundamentally different from the essentialist process of selection.The notion of static patterns is replaced by the notion of populations with high variability.New mutations are constantly occurring, some better and some worse than the common individuals that exist.Since this type of variation in human populations has long been known, it is difficult to understand why the idea of ​​populations was so little known before Darwin, and why it took so long for it to become generally accepted after Darwin.Population thinking didn't really exist before 1800.Even a vehement anti-essentialist like Lamarck thinks only of (identical) individuals and not of variable populations of different idiosyncratic individuals.Natural selection is completely incomprehensible to Lamarck and the orthodox essentialists. Until now, many scholars do not understand the population nature of natural selection.This is a statistical concept.Having a superior genotype does not guarantee survival and reproductive vigor; it only provides a higher probability (指得).Yet accidents, disasters, and other random disturbances are so numerous that reproductive success is not automatic.Natural selection is not deterministic, so it cannot be completely predicted.This has been analyzed very thoroughly by Scriven (1959), but remains a problem for philosophers trained in the essentialist tradition.Evolutionary theory, on the other hand, is capable of making extremely probabilistic predictions (Williams, 1973a). In keeping with Darwin's usual strategy, he advanced a number of arguments to show that the interpretations of the essentialists and natural theologians were incorrect.All species have room for improvement.He cites numerous instances (:82) of the superiority of introduced species over native species (flora and fauna) to prove this.If the native species were really complete, they would not be so easily eliminated.Thus "very slight changes in the habits or structure of a single habitant can often give it an advantage over other animals." Natural selection is of course powerless without abundant intraspecific variation: "Natural selection will have nothing to do unless beneficial variation actually occurs" (p. 82).Darwin placed special emphasis on the emergence of useful mutations.Since the mutations beneficial to man arose in domestic animals, Darwin posed the question: "In the acute and complex struggle for existence, other modifications, which in one way or another are advantageous to each living being, have sometimes occurred during the course of thousands of generations. …难道能够设想这是不大可能的吗？如果这种情况确实发生了，我们能够怀疑（应当记住出生的个体总是比能存活的要多得多）比其他个体具有某种优势（无论是多么微小的优势）的个体会有更多的生存机会和繁殖同种的机会吗？”（80－81页）。这促使达尔文提出下述定义：“我把这种保存有利的变异与排弃有害的变异的现象称为自然选择”（81页）。变异及其遗传属于遗传学范畴，关于达尔文在这方面的假说与学说将在第十六章中详细分析。 值得注意的是，在控制自然选择的众多因素中，达尔文仿效莱伊尔，总是考虑生物性因素（竞争物种之间的相互作用及其相对频率）比物理环境更加重要。因此，“某些栖局动物数量比例的变化（与气候本身的变化无关）将极其严重地影响其他很多物种”。 （81页），达尔文还充分认识到一个被许多后来的学者所忘掉的事实，即不仅成年（成熟）的表现型是选择的目标。“自然选择能在任何年龄（阶段）作用于和改变生物有机体，办法是通过在那个年龄积累有利的变异，并在相应的年龄显示它们的遗传性”（86页）。“在社群动物中，它（自然选择）为了社群的整体利益而改变每个个体的结构” （87页）。 社会史家时常提起达尔文的自然选择进化学说是受了19世纪前半期英国社会与经济形势的影响。这种论点所依据的逻辑是，自然选择是生存竞争的结果，而关于生存竞争据说达尔文是从马尔萨斯那里学来的。更明白的说就是指达尔文学说是伴有残酷竞争，苦难、贫困和为生存而斗争的工业革命的产物；或者是民主制度取代封建专制制度的产物。这些看法果真正确合理吗？Young（1969）和其他人极力想证实达尔文主义是马尔萨斯主义的产物。这些作者之中有些人甚至懒得将达尔文主义分成几个组成部分，虽然他们又一致同意自然选择概念是“来自对种族、国家和阶级形式的冲突与战争的关注” 以及“达尔文主义是将社会科学概念运用于生物学”（Harris，1968）。遗憾的是，所有坚持这种论点的人都只限于这样的泛泛一般的议论。反之，一切严肃的、透彻分析过达尔文学说的来源的达尔文主义者（最近的有Herbert，Limoges，Gruber，Kohn，Mayr）都同意马尔萨斯对达尔文的影响非常有限（“一句话”）和极其特殊这一观点。达尔文和华莱士从马尔萨斯那．里得到的是“人口算术”而不是他的政治经济学。某些马克思主义者声称，“达尔文与华莱士将自由竞争资本家的气质从社会扩展到整个自然界，来把工业界新巨头通过无限制的斗争取得进步的幻想当作世界观。”这种说法并没有得到任何证据的支持（Hodge，1974）。的确，达尔文并不是生活在象牙之塔中；他必然会看到当时他周围的一切；他阅读一切有关的资料（Schweber，1977；Manier，1978），这都可能促使他接受某些现点。然而，如果自然选择学说是工业革命时代精神的逻辑和必然结果，那它就会被达尔文的同时代人广泛而且热情地接受。实际上情况恰好相反，达尔文的学说几乎被人们一致反对，表明它并没有反映时代精神。 正如我对达尔文学说八个组成部分的分析所显示的，其中没有一个是马尔萨斯原有的，所有这些都是达尔文在早先的广泛阅读中遇到了的，多数还是反复遇到的。生存竞争从古希腊人到霍布士、赫德、德坎多尔和莱伊尔一直被人们议论着，虽然没有人比马尔萨斯更强调它的激烈程度。文献资料中广泛地讨论人口过剩的各种限制方式。个性（种群思想）概念是和马尔萨斯完全无缘的，当然，没有这个概念自然选择也是无法想像的。那末，为什么当达尔文读到马尔萨斯关于人口的潜在几何级数增长的评断对对他产生了如此深刻的影响？答案是达尔文是在他的某些其他想法已经成熟到一定程度时读到它的，此时高繁殖力就具有了新的意义。 从达尔文的笔记中已有充分证据表明在1838年9月之前的半年，他的想法已有相当转变。在阅读动物育种家着作的影响下，达尔文开始从本质论思想转向种群思想。在他的较早笔记中，他被模式概念将变异、竞争、灭绝等运用于物种或端始种（例如，运用于反舌鸟的变种）。育种家的议论使他第一次认识到个体变异的极端重要性。在他的第三个笔记本中（D：132），仅仅在他提到马尔萨斯的着名段落（D：135）的前几页，他强调指出个体变异使得“每个个体都成为自然发生（的本源）。”这正当达尔文突然了解到竞争不仅表现在物种之间而且确实也表现在个体之间的时候，正是因为有了这种个体变异才有了自然选择。 如果硬要说达尔文“受惠于马尔萨斯”，则其令人啼笑皆非的一面便是达尔文运用新见解得出了与马尔萨斯完全相反的结论。马尔萨斯的主要论点是为了驳斥Condorce以及Goodwin的人是可以无限度地改善的主张。达尔文运用种群思想得出了与马尔萨斯恰好相反的结论。更令人啼笑皆非的是马尔萨斯充分了解育种家由于人工选择所取得的成就：“（我被告知）在家畜育种家中有一句格言，即你愿意培育到什么样的细微区别程度都能办到…某些家畜的一些后代在很大程度上能具有其双亲的优良特性”（Malthus，1798：163）。但是马尔萨斯利用这一段话至低限度正好否定了无限度的改善的论点。 对他和莱伊尔来说，要接受超越模式界限的观点是无法想像的。因为他们两人都认为个体是基本相似的。再重复一遍，马尔萨斯的作用显然就像将一粒结晶扔进饱和溶液中去。 如果达尔文在这个时刻读了强调高繁殖力及其后果的富兰克林的小册子或某些博物学文献资料，很可能这些文章将会像马尔萨斯的那句话一样地触动他。这是一个很清楚的例子说明一个有思想准备的人能见到他在无准备时所见不到的东西。 有些社会学家还推论达尔文曾受惠于斯宾塞。这种说法更是毫无根据。当斯宾塞开始想到进化时（1852）达尔文的进化学说已基本完成。另外，斯宾塞的一些观点，连同其对目的论原则和拉马克式遗传的依赖，和达尔文的进化观点是完全不相容的。正如Freeman（1974：213）所正确总结的：“达尔文和斯宾塞的学说在来源上彼此无关，在逻辑结构上明显不同，而且在对拉马克式遗传的假定机制的依赖程度以及承认“进步” 是“不可避免的”方面也完全不一样。”将斯宾塞的进化主义与达尔文的等同起来的误解对人类学和社会学的发展都是严重的障碍。 达尔文学说在随后的80年中所遇到的极大阻力无可置疑地证明了要将其八个组成部分恰当地集成起来是多么不容易。它和物理科学中的很多发现不同，在物理科学中同一项发现往往是由几个人同时完成的，因为他们就好像是在拚板游戏中寻找那最后的一块拚板（Merton，1965）。而另一个不知道达尔文研究工作的人要提出相同的自然选择进化学说似乎是完全不可能的。这个学说是如此新颖，和以前任何人对任何一件事所料想的相反，它几乎又过了一百年才被人们普遍接受。在考虑进化问题的人非常少的时候竟然有另一个人在相同时间提出基本相同的学说是完全出人意料的，然而这种事却确实发生了。 达尔文于1858年6月收到华莱士的论文（见第九章）这件旧事引出了很多问题。难道达尔文写这样一封信给莱伊尔就证明他是正确合理的吗？达尔文在信中是这样说的，“我从来没有见到过如此的巧合；如果华莱士手头有我在1842年写出的手稿大纲，他也不可能写出更好的简短摘要。甚至他所用的术语也成了我的书中各章的标题。”华莱士的学说难道就和达尔文学说几乎完全相同吗？华莱士是怎样将他的那些拚板拚成学说的？ 他是按达尔文同样的步骤还是按趋同（收敛）过程完成的？ 我们应该记得华莱士从1845年左右就相信进化，1855年发表了他的物种形成的证据。 自此以后他就一直探索促成进化演变的因素。这里要再一次强调的是莱伊尔的《地质学原理》的重要影响。华莱士和达尔文一样，仔细阅读了莱伊尔在该书中反对物种改变的非常有力的论证。达尔文和华莱士的论点之所以相似，大部分原因显然是由于他们两人都试图反驳莱伊尔所提出的各种特殊论点。由于莱伊尔反对进化思想的理由很具体，所以他也就为针锋相对的抗辩或反辩指点了路数（McKinney，1972：54-57）。 尽管不断地考虑这些问题，然而华莱土的想法自1855年以后显然并没有什么进展，直到1858年2月的一天，“那时我在摩鹿加群岛的特雷提市感染了一场很重的间歇热（疟疾），有一天虽然温度在88F，我在发冷的那一阵子裹着毯子躺在床上，物种是怎样发生变化的问题又出现在我的脑际，不知怎地我想起了马尔萨斯在他的人口论里谈到的坚决的抵制；他的这一着作是我在前几年就已经谈过的，在我的思想中留下了长久而又深刻的印记”（Wallace，1891：20）。 和达尔文的情况相似，华莱士是由于想到了马尔萨斯的人口论而突然得到启发。然而仔细研读一下华莱士1858年写的“关于变种无限制地与原种分离的趋势”这篇文章就表明这种相似并不是完全相同。 华莱士非常明确地谈到他的论点：“自然界有一普遍规律，它使很多变种的生命较其余种长久，并产生连续的变异，和原种相去越来越远”。华莱士在这里使用的仍然是模式语言，然而他的结论却十分清楚的和莱伊尔的论断相反，后者声称“变种有严格的界限，它的变化决不能与原种相差更多一点。” 华莱士的分析的最重要方面是他很谨慎地不陷入物种和变种的形态学争议的泥沼，而是将他的结论植根于十分严格的生态学论点上。他断定某个物种的种群大小（数量）完全不由繁殖力决定而是取决于对潜在种群增长的自然遏制。每年必然有大量的动物死亡以保持数量稳定，而且“那些死亡的必然是最弱的，即很小的，年老的、生病的；那些继续生存的只能是在健康和精力上最优秀的，也就是说那些最有能力正常地摄取食物和防御无数敌害的。正如我们开始时谈到的，这是一场“生存竞争”，在这场生存竞争中最弱的和结构最不完善的必然死亡”（56—57页）。 在华莱土文章的前一部分他的着重点是种群大小（数量）的控制、稳定化选择（淘汰）和物种之间的竞争。随后他就“进而讨论变种，在讨论变种肘前面的评论就得到了直接而重要的运用。”在随后的讨论中华莱土把“变种”这词用于变异个体，也就是种群中不具有相同性质的个体。如果某一物种产生了较好的变种，“那个变种必然迟早会在数量上占优势。”（58页）。 奇怪的是，华莱士的阐述由于和达尔文相同的弱点而略输风采。其中仍有很多模式思想，特别是涉及变种的性质时和达尔文相仿，他依然承认当时普遍流传的用进废退观点。和达尔文一样，华莱士曾在一篇文章中明确表示反对“拉马克的假说”，指出那是直接来自莱伊尔。他却严格的按选择论者的词句来解释猫科动物的短而能伸缩的脚爪和长颈鹿的长颈。华莱士十分强调新适应性状的获得完全符合它们是选择结果的解释。他用下面一段文字作为文章的结尾： 我们相信现在我们已经证明自然界中某些类型的变种具有离原种愈来愈远不断进步的趋势，似乎没有理由为这种进步指定特定的界限……这种进步，步子很小、方向不同、然而总是被必要的条件所遏制和取得平衡，单只经受这必要条件，可以保持生存。这种进步被认为能够贯彻到底，以便和生物所表现的一切现象取得一致，诸如它们在过去时代的灭绝和延续，以及它们所显示的在形态，本能和习性上的非同一般的变化（62页）。 让我们在这里试图更详细地比较一下华莱士和达尔文论点的思路。他们两人都是从物种问题出发，或者像华莱士本人在1908年的一篇回顾文章中所说的，是抱着探索“物种变化的可能原因”的想法。虽然华莱士对问题的分析在某种程度上比起物种形成（他或许认为他在1855年已充分讨论过）更侧重于种群生态方面的研究。华莱土非常直接地把进化问题和人类联系在一起，这一点和达尔文十分不同。他曾经有八年时间和土着民族生活在一起，是什么遏制因素使“一切未开化的土着民族的人口保持基本稳定”这个问题长期使他苦思而不得其解。“这些遏制因素（按马尔萨斯列举的是疾病、饥荒、意外事态和战争等等）是使人口下降的原因。这突然使我想到，对野生动物而言，这些遏制因素的作用将更严酷，一切低等动物都比人类增殖得更快，而它们的种群数量一般却保持不变，最适者生存这念头在一瞬间忽然闪过我的脑海”（Wallace，1903：78）。 和达尔文的情况相似，自然选择进化学说的关键组成部分是承认个（体）性。恰好在50年后（1908）华莱士在谈到这一点时说，“就像朋年前达尔文突然想起的一样，必然性（certainty）忽然掠过我的心头，那些年复一年地在这种可饰的毁灭中存活了下来的从总体上来说必然是具有某种细微的优越性，这种细微的优越性使得它们能够逃脱每一次特殊形式的死亡（而绝大多数则在这特殊形式的死亡中死去），也就是说在那着名的公式中，最适者将生存下来。当时我立刻理解到一切生物永远具有的变异性将为之提供所需要的素材。” 正如前面说过的，华莱士和达尔文的解释之间存在着微妙的差别。华莱土显然对马尔萨斯总的论点，尤其是保持种群数量于稳定水平的每年大量死亡，即“坚决抵制”的印象更深。自然选择中的种群思想在他们两人中来源各不相同。就达尔文来说是来自动物育种及分类研究，而就华莱士而言则来自对人类人口与分类的研究。华莱士对研究家畜变种的价值并不重视并断言从观察家畜中不能对“自然状态下的变种作出推论”。由于这个原因以及其他理由，他在文章中并不使用“选择”这个词，而且似乎对这个词总是不那么中意。 虽然在对待问题的看法上有这些细微的差别，但是华莱士完全同意达尔文的最后结论：置身于严峻和经常死亡之中的种群的无限变异性必然引起进化演变。然而就自然选择而言到了适当时候华莱士的想法就和达尔文逐渐发生分歧。例如华莱士于1867年就放弃了用进废退的观点，并且到了19世纪80年代首先积极支持魏斯曼反对任何获得性（状）遗传。华莱士并不认为要将性选择列为单独范畴，更不相信“雌性选择”（见第十二章）。他还认为生殖隔离机制完全是选择的结果。然而当将这种前后一贯的选择主义（consistent selectionism）运用于人类时，华莱士却有些气馁，因为他认为自然选择能赋予原始人这样大的头脑和道义力量是不可能的事，这需要有某种更强大的力量才能办到（Wallace，1870）。 华莱士到了1862年才从东印度回到英国。是他的论文在林奈学会宣读了四年之后。 他从不妒忌达尔文，尽管后来对某些问题的答案彼此看法不同，但他一直是达尔文的一位高尚的赞赏者。后来华莱士由于他自己的研究工作而赢得了荣誉，尤其是通过他那出色的《马来群岛》和《动物的地理分布》专着；这在随后的80年中一直是生物地理学的经典着作。 最经常用来反对或非难一种新学说的手法有两种。头一种是扬言新学说是错误的，另一种则是声称它不是新的，早已有之。出版以后，有一些忠诚于这后一传统的人接二连三地提出发表自然选择概念的优先权。因为本质论者根本无法理解经由自然选择的进化演变，所以在1800年以前单是这个理由就表明这种优先权的说法是不能被接受的。然而在1858年华莱士和达尔文之前确实有一些真正的关于自然选择的设想。 威尔士（William Charles Wells，1757－1817）是一位在美国南卡罗来纳州居住过一段时间的英国医生，在他去世后（1818）才发表的一篇讨论人类肤色变异体的文章中以补遗的形式提出了自然选择学说（Wells，1818）。威尔士和在他以前的很少的几个人一样，谈到黑人比白人对热带病有更强的抵抗力。反之，黑人更容易感染温带地区的疾病。“既然认为黑人肯定比白人更适于抵抗热带气候疾病，那么就有理由推论那些只是近似黑种的人同样会比纯种白人更适于抵抗热带病。”他说黑白混血儿确是如此。 然后他提到动物育种家，“当他们发现了某些个体比一般个体在较大程度上具有他们所要求的品质时，便将这样的一些母畜和公畜两相交配，然后将它们的后代中最优秀的挑选出来作为新种畜并按这种办法一直进行，直到事物的本性所允许的境地。由人为的方法所做到的，看来也是正按自然的方式以同样的效率（虽然更缓慢）进行，以形成适于所居住地区的人类变种。”他宣称这就是人种在世界的不同气候地区发展的方式。 虽然威尔士明确地提出了一个经由自然选择的进化学说，但是这只是在某个物种之内（而且还只限于人类）适应于当地气候的进化；它决不适用于真正的进化、不适用于物种的增殖、高级分类单位的发展和共同祖先学说。 在制定自然选择的进化学说方面最有资格提出优先权的是马休（Patrick Matthew，1790－1874）。他是苏格兰的一个拥有大量财产的地主，受过良好教育，博览群书而且游历甚广。（Wells，1974）。他的进化和自然选择观点发表在他的着作《舰艇木材与树木学》（1831）附录的注释上。这些注释和这本书的主题几乎无关，无怪乎达尔文和其他生物学家在他于1860年《国艺家记事》的一篇文章中再次提出之前都没有注意到。 马休的背景和达尔文的许多地主绅士朋友很相似，对动植物育种很在行。他明确表示育种要取得成功决定于选择（他曾反复使用这个词）最合式的个体。实际上他那本书的主旨就是这原则也应当适用于树木栽培。他对用辞的选择表明他读过Erasmus Darwin，拉马克，马尔萨斯以及劳伦斯的着作。他显然接受了进化学说，而且十分值得注意的是接受了通过共同祖先的进化。“它们（物种）是不是生命本原在环境变更下分出的分枝？” 他认为渐进进化比“完全毁灭并重新创造”（灾变论）更有可能。他反对林奈的通过杂交的物种起源观点，并认为“相同亲本的后代在环境非常不同的情况下，在很多世代中甚至可能变成不同的物种，不能共通繁殖（co－reproduction）”（384页）由马休的下面一段话可以清楚地表明他的想法和达尔文的思想是多么相似： 生物自我调节的适应性能一部分可以追溯到大自然的极其强大的繁殖力。正如前面讲过的，大自然以其后裔所有变种的形式具有远远超过（在许多情况下超过一千倍）填补因衰老死亡留下的空缺所需要的繁殖能力。由于生存的场地有限和已被占据，所以只有比较强壮、更有活力、更能适应环境的个体才能够从大自然的严酷考验中挣扎出来达到成熟；这种严酷考验是大自然用来考验这些个体对她的完善标准和适于通过繁殖传种接代标准的适应能力……这一群个体逐步取得了对其环境的最佳适应，当环境发生变化时，它们就按其感知变化的限度来改变性状以适应这些变化（385页）。 毫无疑问，马休和达尔文在1838年9月28日一样，也具有正确的观点，但是在随后的20年中他并没有专心一意地将这观点转变成令人信服的进化学说。因而他的观点并没有产生什么影响。 也有人提到Prichard，Lawrence，Naudin曾为达尔文倡立进化论作出过贡献，然而他们的论述和马休的比较起来显得理由并不充分而且不肯定。他们之中有的谈到人种的改进，有的言及植物品种的选育，但是都没有将对选择的可能性的理解运用于发展进化学说。 详细指明观点或资料的来源并不是当时的传统。例如拉马克就几乎从来没有引证他所应用的资料的作者姓名。因此毫不奇怪一再有人指陈达尔文知道这些先驱并使用过他们的材料和发现但从来没有表示应有的谢意，但是没有一点证据支持这种说法。有一切理由相信达尔文并不知道威尔士和马休的有关着作，而就Lawrence，Pricchd，Naudin的论述来说，纵使达尔文知道也会由于这些论述太含混，与经由共同祖先的进化学说关系很少而没有引起达尔文的注意。在中达尔文确实很少提到他引用过的文献的作者姓名，然而这是因为他认为这是一份摘要而且将在更详尽的着作中提出详细的参考文献。现在《自然选择》已经出版（R．C．Stauffer编辑，1975），这就更加容易确定达尔文使用过哪些早期的文献，哪些没有引用过。从他的笔记和其他手稿中甚至能更清楚地看出这一点，它们令人信服地证明达尔文既不知道威尔士的着作也未见过马休的文章。 11．3达尔文革命的影响 达尔文革命被称为最伟大的科学革命是有充分理由的。它不仅是某个科学学说（“物种不变”）被一个新学说取代的问题，而是要求人们对世界以及人类本身的概念进行一番彻底的再思考；更明确的说，是要求放弃某些最广泛流行的，也是西方人最珍视的信念（Mayr，19726：988）。和物理科学的革命（哥伯尼，牛顿，爱因斯坦）相对比，达尔文革命提出了关于人类伦理（道德）和最深层信念的深奥问题。达尔文的新模式，就其整体来说，代表了最先进的新世界观（Dewey，1909）。 达尔文所创议的变革的彻底性可由下列达尔文学说的某些侧重哲学本质的方面加以证实： （1）用一个进化、发展的世界取代一个静止的世界（这并非由达尔文独创）。 （2）证明神创论是难以置信的（Gillespie，1979）。 （3）证明宇宙目的论是错误的。 （4）将共同祖先学说一直运用到人类，破除了为绝对人类中心论辩护的一切口实。 （5）运用自然选择这一纯粹唯物主义过程解释世界的“设计”。这一唯物主义过程由非定向的变异和机遇性的繁殖成功之间的相互作用构成，完全冲破了基督教教条的桎梏。 （6）用种群思想取代了本质论。 此外，还必须添上某些哲学方法论的革新，例如一贯运用假说演绎法，（Ghiselin，1969；Ruse，1979a），对预测的新评价（Scriven，1959），并将终极（进化）原因的研究引入科学之中（Mayr，1972b）。 世界对接受这些革命性的概念准备程度如何？或者换一种提法、采纳达尔文的思想论了多少时间？的冲击或影响是空前的。除了弗洛伊德以外，也许没有别的科学家的着作被翻译得如此普遍，评论如此频繁和详尽，并且写了这样多关于达尔文的书。当时的无数季刊、评论杂志都有广泛的评论，绝大多数的宗教或神学刊物也是如此。这类文献是如此之多因而创立了研究这些评论文献的间接文献（如Ellerard，1958；Hull，1973）。还有另一类研究达尔文思想的影响及其在世界各地逐步被接受情况的文献。生物学历史上的其他时期也从来没有过像由于达尔文学说的胜利而被历史家如此热情而又详尽地加以讴歌（Kellogg，1907；Vorzimmer，1970；Glick，1974；Conry，1974；Moore，1979）。 如果了解19世纪中叶社会上对进化的一般态度就可以更好地认识达尔文学说遭到反对的实质。在达尔文之前研究进化，被认为是哲学范畴的一部分。事实上几乎所有认真思索过进化问题的都是神学家或其他非生物学家，这些人基本上都不具备研究如此复杂的生物学问题的能力。既使是最杰出的达尔文的前驱拉马克也没有系统地掌握大量事实来支持他的进化推想或对进化的可能机制作出详细分析。和当时的观念一致，他将其着作题名为《动物学哲学》（1809），实际上它并不是动物学而是哲学。达尔文是按照科学的方法研究进化的第一位学者。他用大量事实来支持他的论点，这样充实的证据使形势发生了根本变化。进化问题的讨论只要是在哲学基础上进行，议论便能够以形而上学的抽象词句道出。出版后就使得这种探讨方式干脆成为不再可能。达尔文明确或含蓄地指出对生物界绚丽多彩的多样性及其别具慧心的适应性有三种也只有三种可能的解释。这一挑战促使每一个善于思考的读者对他的详尽而又洞察入微的分析面临一种局促不安的尴尬境地：在这三种可能的解释中究竟选择哪一个？ 第一个解释是不断的创造，包括在取代已经灭绝的物种和动物区系方面以及创造更加新的调节和适应方面造物主的不断干预。莱伊尔和塞吉威克（Sidgwick）以及其他许多科学家都在一定程度上接受这种解释。这一解释认为每个物种的每一性状都是被神特地创造出来使物种得以适应它所处的环境。这种对世界的有神论解释在1859年可能仍然占有主导地位，至少是在英国。然而这样一种被莱伊尔称为“永远干预假说”（Wilson，1970）甚至对很多虔诚的科学家来说也过于极端，连莱伊尔和阿伽西对之也有怀疑。 这种情况使得他们去探索第二种，自然神论的进化学说：存在某种目的论的进化规律；这种规律是在神（上帝）创造世界时由神规定的，它将导向更加完备（完善），更能适应并保证动物区系在地质时代顺序中有秩序地轮替。它能说明自然界中一切其他的秩序和规律。（Bowler，1977b；Ospovat，1978）。拉马克起初似乎也曾经支持这一学说，但最终发现并没有日趋完备的趋势。随着生物学知识的增长，这一学说所遇到的困难也愈多。达尔文在中的大多数立论都是指向动物与植物区系分布格局和形态学趋向的明显无规律性，这种无规律性使得按进步规律所作的任何解释完全失效、完全落空。 再也没有别的现象比产生新种来代替由于灭绝而消失的物种的现象更使自然神论者更难堪、更狼狈。新种是由上帝创造的被认为是天经地义的事。而对Herschel和Whewed这样一些科学家来说将新种的产生归之于神迹当然是无法接受的。因此自然神论者便虚张声势地将它们的起源归之于上帝规定的、支配新种形成的“中间原因”