Chapter 29 Chapter 16 The Essence of Inheritance-1

All students of nature perceive intuitively that there is a contradiction or conflict between the phenomena of heredity ("She is very much like her mother!") and the phenomenon of variation.Heredity implies continuity and invariance; variation implies change and difference. When a breeder crosses animals or plants, he often finds unanticipated variants in the offspring.Unexpected variations are often found even when siblings are compared.Finally, an important question arises: Where did this new mutation come from?The source of variation was a key question in biology until Darwin proposed the theory of natural selection.Natural selection can only be achieved when the source of variation is very sufficient (and this source must be continuously renewable).How can this be reconciled with genetic invariance?

The traditional answer is that heredity is not necessarily fixed, that heredity is not completely "hard".In fact, some traits of an individual may be very similar to its father or mother, or its grandparents or earlier ancestors.All animal breeding is based on the existence of this type of hard inheritance.However, if heredity were entirely rigid, variation would be impossible.Therefore, according to the reasoning, there may be two sources of non-exclusive variation; or some inheritance is soft, that is, susceptible to the different effects of each complement, or the genetic material is hard, but has the ability to occasionally produce new mutations.The question of soft inheritance and the source of genetic variation continued to be debated throughout the 19th century and the first three decades of the 20th century.

16.1 Darwin and variation One of the two cornerstones of Darwin's theory of natural selection is the assumption that there is an inexhaustible source, or supply, of variation.Each individual is unique and different from other individuals: "Such individual differences are very important to us because they provide material for natural selection" ( ).But where does this variation come from?What are their roots? Darwin racked his brains on this question.The importance of variation in Darwin's thought is evidenced by the nine hundred pages of his Variation of Animals and Plants Under Domestic Conditions (1868).He once planned to write a book on the corresponding variation in nature, but he was too scared to write because there were too many materials.He condenses a great deal of material on variation into the first two chapters (59 pages).Modern authors of Darwin (eg Ghiselin, 1969; Vorzimmer, 1970; and authors of certain journal publications) are also well aware of the significance of variation.For Darwin, heredity itself and its laws were far less immediate and important than variation and its causes.

Even until now nothing was fully understood about the variation and its causes. The question was shrouded in confusion throughout the mid-nineteenth century.One can understand how complex and difficult the problem is when one realizes that Darwin spent his entire life focusing on it and wrestling with it and still being puzzled.It can be seen by reflection that only after the rise of genetics (such as the difference between genotype and phenotype) can most of the problems be clarified.Also, some confusion has arisen from the lack of consistent application of population thinking. One of the most notable aspects of Darwin's confusion and misunderstanding about variation is that it did not prevent him from developing a well-established and indeed stunning theory of evolution.Only two aspects of variation mattered to Darwin: (1) it was abundant at any one time, and (2) it was fairly "hard-formed".Instead of wasting energy and time on problems that could not be solved at the time, he treated variation as a "black box" for most of his research work

(black box).It exists forever and can be used in the theory of natural selection.Darwin only occasionally studied the contents of this black box, the cause of variation, with little success (as in his theory of pangenesis, see below).Fortunately, the contents of the black box need not be studied for the answers to Darwin's main concerns, such as individual victories in the struggle for existence.It can be deferred until a more appropriate time.One of the secrets of success in science is to choose "solvable" problems (Medawar, 1967). Two problems in mutation particularly troubled Darwin.

(1) The difference between the variation within the field and the variation between the fields.Unfortunately, Darwin never distinguished individual variation from geographical variation, which (especially after the 19th century) seriously affected his discussion of speciation (speciation) (Mayr, 1959a; Kottler, 1978; Sulloway, 1979; see also Part II).This also influenced his inquiry into variability under domestic conditions: "The differences between individuals in earlier domesticated plants and animals ... are generally more marked than the differences between individuals of any one species or variety in a natural state" (;7).

In fact, the individual (ie, within population) variation of strains, varieties, and clans of domestic animals and cultivated plants is often very small, and the ideal of breeders is to breed uniform varieties.Darwin was primarily concerned with the magnitude (range) of variation within domestic species as a whole, that is to say between populations.However, individual variation increases when breeders do not engage in stabilizing selection. (2) There are two completely different intraspecies variations.Breeders and naturalists (in fact, anyone who deals with variation) until the first two decades of the 20th century believed that there were two different kinds of variation, discontinuous and continuous (also known as individual variation).Discontinuous variation is the variation exhibited by all variants that differ markedly from the "pattern" without a series of graded intermediaries between these variants and the "positive pattern"; albinos are an example of discontinuous variation.As far as essentialists are concerned, any new thing can only originate from a sharp deviation (mutation or sudden change) from the pattern, so discontinuous variation occupies an important position in the evolution theory of essentialists (see Part II).

Although Darwin recognized the existence of discrete variation as another category, he did not consider it to be evolutionarily important. Unlike other early scholars, he emphasized the universality and biological significance of individual variation or continuous variation. Where did Darwin get this all-important new insight?He drew his lessons chiefly from a study of the writings of animal breeders; these breeders, beginning with Bakewell, Sebright, emphasized the uniqueness of the individual which made selection and breed improvement possible.This impression was reinforced by Darwin's own taxonomic studies, in which, like taxonomists before him, he found that no two individuals were identical when looked at closely.

It is this individual variation (Darwin never tires of emphasizing it) that provides the material for selection and thus creates the conditions for evolution.Darwin himself was very unclear about the nature of this continuous variation, and this issue has always been controversial. Through the research of Nilsson-Ehle, East, Baur, Castle, Fisher and other geneticists, the problem was not resolved until after 1910 (see Chapter XVII). Those who disagree with Darwin raise two objections in particular.First, they claim that until Johnson and later, such continuous variation merely reflected phenotypic plasticity and was not heritable.Another of their objections, going back to Leyle and before, is that such continuous variation is strictly bounded and must never cross "mode" boundaries.Both of these objections were later denied, and the extreme importance of individual variation is no longer in doubt today.Moreover, as we shall see below, genetics turns out to show that there is no real difference between the genetic basis of continuous and discontinuous variation.

Darwin was a product of his time, and he had to get to the bottom of everything. It was not enough for Darwin to simply recognize that there were a lot of mutations.Variability must have an ascertainable cause.He doesn't believe in "spontaneity" (spontaneous) variation. "I do not believe that variability, as some scholars have imagined, is an inherent and necessary accident that applies to all living things under any circumstances" ().Darwin considered the most important cause of variation to be various influences on the parental reproductive system of an individual, especially shocks or major changes in the environment.These effects, he argues, do not produce special variants or directed variation; they merely increase variability in offspring, giving natural selection a greater latitude to operate.

Darwin occasionally admitted that he had sometimes been careless in referring to variations as "due to chance. This is, of course, a wholly false expression, but it just shows our frank confession of ignorance of the causes of each particular variation." ().Darwin's confidant Hooker saw better yet that there is no need to establish a causal connection between particular conditions and particular variations which occur under those conditions. "I am inclined to attribute the least (individual) variation to an inherent tendency to change; this principle has nothing to do with the physical environment (conditions)" (Letter to Darwin, March 17, 1862; 1918, I: 37 ).In Darwin's view, the "inherent tendency to change" may have the same tone as Lamarck's "inherent tendency towards perfection".It was not enough for Darwin to admit that genetic variability was just another manifestation of the imperfections of nature, and both Darwin and Hooke were vague about the real issue on which they disagreed.Is the variation they refer to a process or the result of this process?Also, what does chance mean?It is impossible to accept the interference of randomness in that era when only the process of obeying the "law" can be respected in science. Darwin's opponents, in spite of all Darwin's denials, clung to his statement that his mutations were due to chance, as little as anything else.In fact, this debate continues today in various forms (such as "Are mutations random?").What opponents of Darwinism fail to understand is that Darwin and his followers never doubted the true physicochemical cause of all variation; they simply denied that variation had a teleological element.Inherited variation is not a specific response to adaptive needs. It is particularly difficult to tease out gradual changes in Darwin's thinking about the causes of variation, as they relate to changes in his ideas about the causes of adaptation (natural selection) and the nature of heredity (soft or hard).Those who do not believe in natural selection are bound to rely on soft inheritance, and must argue that adaptive responses to environmental demands do exist.Once Darwin adopted natural selection as the mechanism of evolutionary change, all he needed was a process or processes that produced variability.If, however, heredity is generally of the hard type (as we shall see below, Darwin arrived at this conclusion), that is, if the characters of the parents are generally transmitted unchanged to the offspring, special stimuli are required to bring about their occurrence. Change.And since offspring are the products of the reproductive system, the stimulus must affect the reproductive system.This chain of arguments is perfectly logical. It appears that Darwin had evidence to support his thesis.He observed in nature that there are both extremely variable species and uniform species, so he came to the conclusion that there must be some factors that can affect the variability of species.On this point he recalled that domestic species, such as the breeds of dogs and the varieties of cabbages, were each supposed to have descended from a single progenitor species. He also observed that "the conditions most favorable to variation seem to be the breeding of the organisms concerned for many generations under domestication" (1844: 91).What aspects of domestication conditions are associated with increased variability?What factors drive changes in a genetic structure that is generally so stable?This is "simply due to the fact that our domestic organisms have been bred under conditions of life so different from those of the parent species" (: 7).Darwin did not suggest, as might be assumed from reading this passage, that life in a different environment induced new characters directly, but only that some factor, perhaps "a surplus of food," increased the variability itself.And Darwin also thought that the increased variability was due to the fact that males and the male reproductive system are apparently more susceptible than other parts of a living organism to the effects of changes in living conditions" (:8, see also other similar remarks in Darwin's writings). An important difference between this explanation and the one in favor of soft inheritors is that Darwin's variation was not directed in the direction of the environment or the needs of the organism.The directional tendencies observed in evolution arise from additional causes: "Continuous accumulation by natural selection . . . has caused major changes in structure" (: 170). Many statements scattered throughout Darwin's writings imply that genetic material is generally immune to environmental influences.Before the 1870s, Darwin was actually the only one who realized this. It can be seen from Darwin's notebooks that when he began to think about the problem of evolution, he forged an indissoluble bond with the problem of heredity, but he rarely talked about it in the book.He did think at the time that most individual variation was heritable. "Perhaps the correct way of thinking about the whole problem is to regard the inheritance of any one character as the rule, and the non-inheritance as the anomaly" (: 13).It is clear that natural selection cannot favor traits that are not inherited, so "any variation that is not inherited is of no importance to us".It was not until Variation of Animals and Plants Under Domestic Conditions, published in 1868, that he published his views on heredity in the form of his pangenesis hypothesis.This point I will introduce below, but for the sake of analysis, I will point out two points in Darwin's genetic theory that historians of science have not yet reached full agreement.The first point is whether Darwin believed in fusion or granular inheritance.Since the nature of this controversy can be fully explained only in terms of Mendelian inheritance, it will be discussed after the introduction of the rediscovery of Mendel (see Chapter 17).A second disagreement concerns the extent to which Darwin believed in various forms of soft inheritance, especially inheritance of acquired traits. The idea that the environment or "use it or lose it" (or both) can affect the inheritance of traits was widespread almost until the end of the 19th century (Zirkle, 1946), and was held by many biologists even into the 20th century . (Mayr and provine, 1980).This view is usually expressed in terms of "inheritance of acquired traits", but this expression is not accurate, because this view usually also includes genetic material being changed by climate or other environmental conditions (Jeffreyism), or directly affected by nutrition. Conditional effects are not necessarily mediated by phenotypic traits. The "Bible" (1 Moses: 30) records the story that the experience of the pregnant mother can affect the offspring, and the literature on teratology also recognizes this point of view and regards it as the main reason for the occurrence of freaks.These conditions are often interpreted as non-inherited phenotypic changes. The basic concept underlying this view is that the genetic material itself is soft and malleable, or "soft."In terms of this theory it doesn't matter if the genetic material changes slowly or quickly, whether it changes directly or via an "acquired trait", what matters is that the genetic material is not considered fixed, not unchangeable, not "hard-formed" .Strangely, soft inheritance has been generally recognized and considered an axiom until after 1850, when no one tried to prove its validity and explore its mechanism.Darwin, Spencer, and Haeckel were the first scholars to attempt this work. (Churchill, 1976).Apart from a few neglected pioneers, it was later suggested that only hard inheritance might be the only mode of inheritance (see below). At the end of the 19th century, among the neo-Lamarckists, Lamarck was considered to be the first to propose "inheritance of acquired traits". In fact, acquired trait inheritance was a widely popular concept in the 18th century, and all famous biologists of that period, including Buffon and Linnaeus, held this view.Blumenbach, for example, believed that the dark-skinned race was formed by the intense tropical sunlight acting on the liver of the fair-skinned race.When the liver is exposed to strong sunlight, it darkens the bile and deposits the pigment in the skin.People who are more experienced in racial issues than Blumenbach, such as Herder, can easily deny his opinion, pointing out that the skin of whites and their children living in tropical areas has not turned black, while African slaves The skins of the descendants of the genus, though they have lived in temperate regions for many generations, are still black; and if any marked change in the color of the skin is found, it is the result of interbreeding between the races.No one, however, has more thoroughly denied the influence of climate on racial traits than Prichard (1813).He concluded that "the color acquired by the parents in a hot climate is not passed on to their offspring, and thus does not take part in the formation of natural varieties." The inefficiency of climate is likewise demonstrated in animals.Animals that have been bred for generations in zoos or cages do not change a bit in their appearance.Although the concept of soft inheritance was pointed out early on as unfounded, most scholars have stuck with it. The only concession made by its proponents to the counterargument is to assume both soft and hard inheritance. The admission of unchanging essences (the essentialist's basic tenet) seems to necessarily require belief in hard inheritance.So it amazes me how easily the essentialists of the time were able to reconcile soft inheritance with the notion of an unchanging essence.Their way around the dilemma is to attribute to "chance" all characters that are inherited softly, and their variation does not affect the essence.Agassiz (Louis Agassiz) claimed that part of the inherent potential of the essence can change in response to the environment, even "predictability".More die-hard essentialists look around for examples of climate change without long-term consequences (such as human migration).For exampleC． F． Wolff was satisfied to learn that certain plants transplanted from the west of Russia (St. Petersburg) to Siberia, though greatly altered, returned to their original form when their descendants were brought back to St. Petersburg.In his opinion this proved that the influence of external factors did not go deep into the essential structure of biological organisms. (Raikow, 1947; 1952).This is likely to be the basis for the hard genetic theory, but it has not been pursued. Once the theory of evolution is put forward, the question of whether soft inheritance exists becomes crucial.Is evolution due to the inheritance of acquired traits, as Lamarck contends?From 1859 onwards, transplanting plants from lowlands to high mountains and back again became a popular method for testing environmental effects at that time, and both Bonnier and Kerner had adopted them.However, this method is not very satisfactory, because the individuals of most lowland species cannot tolerate the alpine climate, and if a species with an alpine ecotype is selected, it must be very strict to prevent the transplanted plants from mixing with the local ecotype; therefore Kerner "does not found no instance of hereditary variation in form and colour" has little effect on the conclusion.This problem was finally resolved in the 1930s by the studies of Clausen, Hieser, Keck et al., but by then it was no longer necessary to deny the inheritance of acquired traits. Darwin believed in both soft and hard inheritance throughout his life, only changing his views on the relative importance of the two.Soft genetics clearly prevailed in his early notes.He even notes (though he is not fully convinced) that in Interracial crosses the paternal line may influence subsequent pregnancy (B:32, 181; C:152) and that the "wishes of the parents" may influence the offspring ( B: 219).Most of his remarks are rather vague so that they can be interpreted both as non-genetic changes and as effects on later generations. (B: 3, 4; C: 68, 69, 70, 195, 220).Even at that time Darwin categorically denied that drastic changes in limbs (such as disability) could have genetic effects (C: 65-66, 83; D: 18, 112). More than 20 years later, Darwin no longer mentioned the suspicious legend of breeders in his studies, and adopted natural selection as the driving force of evolution, mainly relying on hard inheritance.A careful reading of Darwin's writings, however, reveals that he still occasionally cites what appears to be evidence in favor of soft inheritance.He recognizes three possible sources of this type of variation.The first is that the effects of environmental changes increase variability through the reproductive system, a source that is compatible with hard inheritance. The other two require the recognition of soft inheritance: the direct effects of the environment and the effects of use or disuse. Darwin believed that environment was one of the factors that could cause variation.He repeatedly mentioned in "climate, food, etc. may have some small and direct effects" (pp. 85, 15, 29, 43, 132).Darwin also frequently discussed the diversity and high variability of domestic animal and cultivated plant breeds.He attributed the increased variability to altered, especially favorable, conditions of life.In fact interbreeding in cultivated plants is the chief cause of increased variability (perceived to some extent by Darwin), while in some breeds of domestic animals the reverse is achieved by frequent inbreeding (breeding) Epistatic systems that disrupt equilibrium to increase variability (Lerner, 1954).Darwin also tended to likewise stress that such direct effects "are of quite minor importance compared with the effects of natural selection" (p. 209). The notion that "living conditions" are not important in generating new variations is in the origin of species.There are also expressions on pages 10 and 134 of .Darwin put it more clearly in his letter to Hooke (L.L.D. 11:274): "My conclusion is that the role of external conditions other than mere variability (mereVariability) is very small. I think that This mere variability (making the offspring less like the parents) is very different from the formation of distinct varieties or new species... I thought the formation of distinct varieties or new species was almost entirely due to natural selection, which may be incorrectly called chance variation or variability".Since Darwin did not clearly distinguish between phenotype and genotype, in all the examples he cited, it was impossible to explain whether the variation caused by the environment he considered was hereditary or non-hereditary. Perhaps no other period was so indifferent to the direct effects of circumstances as Darwin wrote. But in 1162 when he had finished the first volume of Variations of Animals and Plants under Domestic Conditions, he wrote to Hooke, "My present work leads me to believe that the direct action of physical conditions is important." In 1878 he admitted , "I may have underestimated the power (of external conditions) in previous editions" (see Vorzimmer, 1970: 264). In a letter to Galton in 1857, he wrote, "Year after year he and I have paid more and more attention to this effect (changes caused by use and disuse during the individual's life)." Of all the phenomena which Darwin considered conducive to the demonstration of soft inheritance, none seemed to him more important than the effect of use or failure.He realized this while studying domestic animals. "There is no doubt that in our domestic animals certain organs are strengthened and enlarged by use, and shrunk by disuse; and such changes are hereditary" ( 134).Darwin was so keenly aware of the importance of this factor that he devoted a whole section of Chapter 5 to it.He discusses it with examples such as the atrophy of the wings of flightless birds, the disappearance of the combs of the dung beetle (dung beetle), the winglessness of some beetles in the Madeira Islands in the Atlantic Ocean, the atrophy of moles and other burrowing mammals. Vision is degraded, and burrowing animals lack eyes and pigment.With regard to generally underdeveloped organs, Darwin says, "I think that the chief agent of their formation is disuse" (p. 454).His emphasis on this factor (use it or lose it) is evidenced by his repeated references to it as an evolutionary agent in , 480 pages).Of course, use it or lose it matters only if you believe in the inheritance of acquired traits.Darwin demonstrated this repeatedly.He has talked about the fact that frequent milking of cows can genetically make their udders larger.Darwin said with absolute certainty: "Change [caused by use and disuse] is hereditary" (p. 134). The so-called use-it-or-lose-it effect is not difficult for modern evolutionists to explain; it results from a relaxation of stabilizing selection (often reinforced by counter-selective forces).Although Darwin recognized that selection was involved in the formation of underdeveloped organs, he was not prepared to go to the extreme by relying solely on selection to explain underdevelopment. Darwin's thought was still largely constrained by concepts that preceded him, and he sometimes explained observed phenomena in terms of use-it-or-lose-it, which appear to us today to be "obviously" due to natural selection.Darwin found, by careful measurements, that "in proportion to the whole skeleton, the wing-bones of the domestic duck are lighter, and the leg-bones heavier, than those of the mallard" (p. 11).Curiously enough, Darwin did not attribute this to natural selection under domestic conditions but assumed that the change was partly non-hereditary, corresponding to differences in plants grown in different soils; Fly less and walk more" (p. 11).His experience with plants and plant breeders led him to believe that animal phenotypic plasticity is much greater than actually observed. Another line of evidence also shows that Darwin recognized soft inheritance.He believes that if organisms are kept in the same environment for a long time or if a certain structure is continuously used, the genetic basis of traits will be strengthened: "If the variety is vigorous for a long time, its genetic basis will become stronger and stronger".When considering "whether certain effects become inherent and heritable at all and not others", he concludes that "it can only be those which have to act in the same way for many successive generations" (C : 171), and "the longer an organism flourishes, the more resistant it is to any change; the shorter it is, the less resistant it is". (D: 13, 17).It was learned a century later that this was the result of stabilizing selection. From this Darwin concluded that the older the breed or geographical variety of domestic animals, the greater was its influence in cross-breeding. He dubbed this phenomenon "Yarrell's Law," after a friend of his, an animal breeder, William Yarrell.Darwin's above conclusions obviously come from him (C: 1, 121; D: 7-8, 91).Yet Darwin also admitted that this law sometimes does not work (E:35). On the other hand, if a certain trait is under unfavorable conditions for a long time, it will also weaken itself.He argues that "if we ... grow some varieties of Chinese cabbage in poor soil over many generations ... most or all of them will revert to the wild original species" (: 15).Views like these were the most widely prevalent among animal and plant breeders at the time. Some recent historians agree with Darlinston (1959) that Darwin was Inheritance of the hard form was known only in the first edition (1859), but after reading Jenkin's review (1867) "returned to the inheritance of acquired characters".As pointed out by Vorzimmer (1963; 1970) and others, and from the above analysis of Darwin's soft genetic view, it can be seen that Darlington's view is completely wrong.Admittedly, Darwin did concede slightly to soft inheritance later in life than he did in 1859, but it never became a major part of his explanations.Whenever he compared the roles played by inheritance of acquired traits and natural selection in evolutionary change, he always made it quite clear that he had always considered natural selection to be the determining factor. Chapter 27 of Darwin's Variations of Animals and Plants under Domestic Conditions (1868) is entitled "Provisional Genesis Hypothesis".He proposed this hypothesis because "it may be useful to connect many facts which as yet have no valid reason to connect them" (1868: 357).In the subtitle of a chapter Darwin enumerates "the facts connected by a single point of view, viz. the different types of reproduction, the direct action of the male factor on the female, the development, the functional independence of the various units or factors of the body, the variability , inheritance, back mutation (atavistic inheritance)". No single doctrine can provide all the answers this ambitious vision needs.Darwin's theory of heredity (which he himself somewhat misleadingly called the pangenesis hypothesis) was really a whole set of theories. The first theory is that the transmission of heritable properties and the guidance of development are caused by very small, distinct particles, so-called "microbuds".Each type of cell in the body has its own microbud; the mosaic of traits in hybrids is the result of intermingling parental microbuds; the phenomenon of reversion to ancestral traits so attractive to Darwin is due to the activation of previously dormant microbuds reason. As de Vry first stated in concise language, Darwin's genetic theory, which proposes that various traits of biological organisms have their own individual and independent particle basis, is the first comprehensive and consistent content. genetic theory.It can explain a large number of observed phenomena; and it is also a historical fact, because all subsequent genetic theories, especially those of Galton (Galton, 1876), Weismann (1883-1892), de Vry (1889), are all Influenced by Darwin's theory of genetics.It can explain (not very different from Mendel's subsequent explanation) "prepotency" (dominant) and "reversion" (recessive), regeneration, and other genetic and developmental phenomena. As stated earlier, this theory cannot explain the inheritance of acquired traits.How can the influence of use, waste, and retreat on peripheral organs (hands, skin, eyes, brain) be transmitted to reproductive organs?To illustrate this problem, Darwin proposed the "transportation hypothesis" (Devry called it so).Cells are able to shed microbuds at any stage of their life cycle, "they (microbuds) circulate freely in the system, reproduce by dividing themselves when provided with proper nutrients, and subsequently develop to resemble the cells from which they originate Cell". (Darwin, 1868:374).This cycle of microbuds is the second part of Darwin's theory of heredity; it enables microbuds to accumulate in the sex organs, or to concentrate in the buds of plants.Finally, "In variations caused by the direct action of changing environmental conditions...on the principle of pangenesis, the tissues of the body are directly affected by the new conditions, thus throwing off altered sprouts; these altered sprouts, together with its newly acquired traits are passed on to future generations” (pp. 394-395). This is pangenesis in a narrower sense, and it is what critics of Darwin's theory refer to when they refer to Darwin's pangenesis. The idea that matter is transported from the body to the reproductive organs was not first thought of by Darwin, and Zirkle (1946) can list 90 pioneers from Hippocrates (see also Lesky, 1950).Darwin himself (1868:375) also mentioned similar theories of Buffon, Owen, Spencer and Bonnet, etc. and pointed out where his theory differed from theirs. Darwin seldom talked about his theory of transport, which he regarded as a "dream" or "stillborn", yet he believed that "it contained great truths".Of course, it was quickly dismissed (see below).Ironically, Darwin's theory was no longer needed 15 years later after Weissmann's denial of soft inheritance based on a series of facts and theories.If there is no inheritance of acquired traits, then there is no need to raise the question of the transfer of genetic material from the body (somatic cells) to the germ cells. Darwin was the first scholar to emphasize the pervasiveness of hard inheritance, but even he did not completely abandon soft inheritance.So who was the first to categorically deny soft inheritance?Every preformationist should be unreservedly opposed to soft inheritance, but I have never found a scholar who explicitly made this point.It has been mentioned that Prichard was the first to publicly deny soft inheritance in the first edition (1813) of his Inquiry into the History of the Constitution of Man. Prichard did deny that climate was related to race differences, but he still considered soft inheritance in terms of culture and other factors, and accepted soft inheritance to a greater extent in later editions of his above-mentioned work. Although Lawrence once said, "The offspring only inherit their (parents) innate characteristics but not any of their acquired traits", however, the causes of birth defects still consider the influence of various factors on the mother, and there are signs shows that he also sometimes believed in soft inheritance (Wells, 1971).The same was true of other scholars until the 1870s.The first scholar who explicitly denied soft inheritance may be Sith: "Before soft inheritance was denied, I always supported the point of view that the characters acquired during the individual's life cannot be passed on" (1874: 158).Later Weisman (1883), Kolliker (1885), Ziegler (1886) and other scholars followed his view (Churchill, 1976). 硬式遗传的先躯及其对立面（例如微尔科）之间的争论表明直到19世纪80年代获得性状遗传仍然是公认的信念，也说明它被当时的有关生命本质的观点支持的程度。 达尔文的表弟高尔敦（Francis Gallon，1822－1911）虽然在总体上反对软式遗传但也可能并不是全然加以否定。在19世纪80年代他对遗传提出了一些具有预见性的观点，然而显然被当时的生物学家完全忽略了，一部分原因是高尔敦的这些观点是在非生物学杂志上发表的，另一部分原因是他的某些最具创见性的想法根本就没有发表。例如他干1875年12月19日在给达尔文的信中对杂种性状的说明就是如此。他在这封信中提出了典型的孟德尔颗粒遗传学说，认为遗传单位不融合但能分离（Olby，1966：72）。然而他对明显的不连续性状（如花中的红花与白花）并不特别感兴趣。他更注意的倒是一般性状，如大小或（人类的）智力。1876年高尔敦发表了他的十分详尽、面面俱到的遗传学说，在这学说中他预见了许多随后由魏斯曼等人发展的观念，包括减数分裂。 高尔敦同意达尔文的“大量的、各具独特性质的有机体单位（Organic units）” 的学说。由于他反对达尔文的泛生论（至少是德弗里称之为“运输学说”的那一部分），所以只注意生物的全部潜力都包罗在受精卵中这个问题。他将这些遗传颗粒的总体命名为（受精卵中的）“定子组”（stirps），它和魏斯曼的种质（1883）以及内格里的异胞质（1884）显然相同。和达尔文相仿，高尔敦对回复到祖先性状以及个体中突然出现双亲所没有的性状这一类现象印象极深。因而他得出的结论（正像在他以前的淖丁在1865年所作的结论一样）是“（受精卵定子组中）大量的胚芽只有比较很少的能够发育，”其它的就处于休眠状态，有时甚至可以休眠好多代（1876）。在讨论性别的意义时，他认为性别的功能是保持遗传上的变异性，也就是说防止基因损失（就像我们现在所说的）。他认为当受精卵是由双亲的贡献共同组成的，这种损失就很少会发生。他意识到细胞核减数分裂的必然性；他远在魏斯曼之前就提出了配子选择（germinalselection）学说（1876：334，338）。除孟德尔以外他和他的同时代人相仿，认为每一遗传定子（genetic determinant）在（他的）定子组中由大量完全相同的复制物（replicas）代表；他还讨论过随机固定（random fixation）以及许多其它有趣的观点。遗憾的是，在斯宾塞传统影响下他主要按“运动和力”来考虑遗传，因而他将个体发育作为遗传的结果来解释令人很难满意（1885年以后他又提出了一个完全不同的遗传学说；见第十八章）。 在个体发育中未耗尽的那一部分走子组从一代传递给下一代。进化演变究竟是怎样发生的并不清楚，即使高尔敦在很多年中否定软式遗传，也是用隐约的言辞暗示：“很可能发生这样的情况，即某些种类的胚芽在很多代中可能并不发育，最后它们可能会发生相当大的变化”（1876：338）。他之所以采取这种解释是因为他承认“结构上的变化可能反作用于与性有关的部分”（348页），然而他反对达尔文的运输学说。为了用实验来否定它，高尔敦对不同颜色的兔进行输血，然后使这些输血过的兔近亲交配；在其后代中从未发现在颜色上与亲代的有什么不同。按达尔文的假说，如果有不相同的微芽随血液循环就应当产生不同颜色的后代。这些实验并没有促使达尔文放弃他的泛生论。 他十分生气地说这些实验仅仅表明微芽是按血液循环以外的途径运输。这种可能性被Castle和Phillips的实验（1909）彻底否定，他们将一只未成熟黑色豚鼠的卵巢移植到卵巢已全部切除的一只白色雌豚鼠中。然后将这只白色雌豚鼠与一未经处理的白色雄豚鼠交配，在连续的三窝后代中全是黑色的。 高尔敦是一位言行与众不同的业余爱好者，在很多领域中他都独辟蹊径。他大力支持种群思想，对个体的独特性比同时代人了解得更清楚。这促使他发现了指纹的特异性和鉴别价值。他推动了种群（群体）统计学的发展（Hilts，1973）；回归与相关这两个重要的统计学概念就是由他提出的。人们普遍地认为高尔敦是优生学的创始人。 19世纪70年代是一个过渡时期。对软性遗传的全面抨击（除某些方面外）已经消散。 在达尔文的泛生论中细胞仍然被认为是生物有机体的结构单位。即使是具有最进步遗传学说观点的高尔敦也无法使细胞学的新发现和遗传学说联系起来。因此他无法为他的推论奠定理论基础。和达尔文相仿，高尔敦并不了解一旦认识到细胞核（而不是整个细胞）是遗传物质的载体后会引出完全新的问题。这时人们必然会问，细胞中的细胞核与细胞质是什么关系？细胞质是否向细胞核、尤其是生殖细胞的核输送了什么物质？ 应当注意的是直到19世纪70年代关于遗传及其物质基础的观念还是非常模糊的。当认识到细胞核是遗传载体和发现了核中染色质的复杂结构后情况就完全改变。种质的精细结构看来并不像是会对一般的环境影响（如气候和营养）作出恰当反应的结构。染色质的精致纤巧结构看来更符合硬式遗传而不是软式遗传。过去一直认为是能证实软式遗传的证据可靠性如何？新证据是否有利于否定它？遗憾的是，高尔敦和达尔文都没有觉察到这一时期冲细胞学在德国的长足进展。 16．2魏斯曼 魏斯曼（August Weismann，1834－1914）不仅首先十分明确地提出了上述问题而且也是对这些问题作出明确答案的第一位学者，他是古往今来伟大的生物学家之一。在上一世纪研究细胞学、发育和遗传的学者中，魏斯曼的特点在于他是一位毫不妥协的自然选择论者。他的进化学说排除了获得性状遗传的一切残余或其它形式的软式遗传，被称为新达尔文主义（Romanes，1896）。 从科学的方法论角度考虑，在他的那个时代他也是以对问题的仔细、理智分析而着称。当他要解释某个现象或过程时，他总是试图推究各种可能的解答，其中几乎毫无例外的包括现在认为是正确的答案。由于那时资料不足、有时甚至只有错误的资料，所以魏斯曼也选择了现在被否定的解决途径。但是这丝毫也不能掩盖或减少他的成就的光彩。 他从来不草率地作出抉择而总是首先全面地探究一切可能的解释。他的遗传学说是第一个真正完整的学说，他的推理为整个下一代的研究铺平了道路。正如Correns 所说，在魏斯曼的铺路工作之后于1900年重新发现孟德尔定律就算不上是伟大的成就了。 魏斯曼（1834年1且17日生于德国法兰克福）在青年时代就热心于采集蝴蝶、甲虫和植物。他先学医并行医几年，后来才转向动物学（组织学）。但改行不久就患了严重眼疾不能在显微镜下从事研究并迫使他半休。这倒使他因祸得福。他从实验研究转向理论研究，将全部时间用来深入思考生物学问题并探索其答案。他最关心的是，经由自然选择的进化，遗传的物质基础，发育机制这三个互相关联的领域。他比他同时代的人看得更清楚关于达尔文主义是否正确的重大争论如果没有一个完整的遗传学说是永远无法解决的。 他的第一篇关干遗传的重要文章发表于1876年，80年代发表了一系列重要论文，最后在1892年出版了他的不朽名着、628页的土种质论。（Keimplasma）。和一切富于想像力的开拓者相仿，魏斯曼十分虚心，当他认为他的学说由于新的证据要求修正时，他从不吝于修订。可惜的是，他的某些修订，尤其是1890年以后发表的，从今天看来并不都是恰当的。 魏斯曼在他于1876年提出的遗传学说中将遗传解释为是由于分子运动的结果，并引用了他所赞同的von Helmholtz（1871）的一句话：“一切定律最后都归结为运动定律。”他之反对达尔文的泛生论并不是由于它支持软式遗传而是因为它基于物质而不是根据运动。魏斯曼在那时仍然相信“外界条件对可以遗传的进化物质产生影响”（1868： 12）。然而他对软式遗传的重视程度显然逐步减少，因为他在1875－1880年之间曾经通过很多实验对它进行过验证。 魏斯曼于1883年和1885年提出的遗传学说不仅和他以前的尝试大不相同而且是真正完整全面的。它由两个新见解统帅。第一个是所有的遗传物质含于细胞核内。正如魏斯曼十分明确指出的，他的学说的“基本观点是，遗传是将具有一定的化学结构、归根到底是一定的分子结构的物质从一代传递给下一代来实现的”（1889；英译本：167）。 另一个新见解是不承认任何形式的获得性状遗传。 要否定获得性状遗传有三种办法。第一种是证明所设想的运行机制是不可能发生的。 这是魏斯曼的主要研究途径。在细胞的结构和细胞分裂中没有什么能使获得性状遗传得以实现。事实上，在某些生物（魏斯曼特地选用了水螅）中未来的生殖细胞在刚刚进行过几次细胞分裂后的动体早期即已隔离并“被搁置在一边”（按日常用语来说）直到繁殖过程开始。对生物有机体其余部分的影响将不可能传递给已隔离的生殖细胞的细胞核。 这一观察结果使魏斯曼在1885年提出了他的“种质连续”学说。这学说指出从一开始“种迹”（germ track）就和“体迹”（soma track）分开，因而体质所发生的情况不可能传给生殖细胞及其细胞核。我们现在知道魏斯曼的基本观点，即种质与它在表现型中的表现彻底分离，是完全正确的。他提出这种分离的直觉也是无可挑剔的。然而在可能实现这一点的两种方式中他选择了生殖细胞与体细胞的分离，而我们现在知道最重要的是每个细胞中细胞核的DNA程序和细胞质中蛋白质的分离。 否定获得性状遗传的第二种方式是通过实验。如果真有获得性状遗状，那么便必然有某种东西从受了影响的部位传给生殖细胞。用进废退的旧学说（达尔文甚至也在一定程度上承认它）可以通过完全不使用某一结构的方法（Payne的实验）进行检验；或者，如果有某一器官将微芽传送给生殖细胞，那末将该器官切除然后连续经过许多世代以后将会使这器官逐渐萎缩。最后，如果植物由于栽培条件不同而发生了可以遗传的表现型变化，那末从纯系（Pure line）的最大、最小个体中进行选择育种就会产生渐进性结果（Johannsen，1903）。从Hoffmann和魏斯曼开始，这类实验一直进行到20世纪30年和40年代，但是实验结果始终都是否定的（另见Galton，Romanes，Castle与Philipps）。换句话说，获得性状遗传学说被检定它的正确性的所有实验所否定。 否定获得性状遗传的第三种途径是指陈那些声称需要用获得性状遗传来解释的现象可以根据达尔文学说同样加以说明或说明得更圆满。从20世纪20年代一直到40年代有关进化问题的文献都是为了解决这个问题。 19世纪70年代魏斯曼一直相信获得性状遗传。是什么确切原因促使他转变观点还不清楚。也不了解魏斯曼究竟是先认识到获得性状遗传不合理然后便采取种迹学说还是先后次序正好反过来。事实是在他的1883年的文章中就已经有不少地方反对软式遗传，因而可以有充分理由认定这一总的认识先于提出一种特定机制。魏斯曼在19世纪70年代已经是一个也许根本不需要一种额外机制的坚定选择主义者这一事实也支持这一解释。 魏斯曼否定软式遗传的这一革命性观念招来了强烈的抵制或反对。它不仅受到19世纪80年代和朋年代势力达到顶峰的新拉马克主义者的攻击，甚至也遭到正统的达尔文主义者（他们仍然接受达尔文一度赞同的用进废退观点，例如Romanes，1896；Plat，1904）的非难。然而魏斯曼的观点却被英国的Lankester，poulton，Thiselton Dyer采纳，一直到20世纪30年代也许他在英国的支持者比他在本国（德国）的支持者还要多。 到了本世纪30年代和40年代，由于进化综合的结果他的观点才被普遍承认（Mayr andProvine，1980）。