Kitabı oku: «Essays Upon Heredity and Kindred Biological Problems», sayfa 25

Nägeli is quite right in maintaining that the solid protoplasm alone, as opposed to the fluid part, i.e. that part of the protoplasm which has passed into solution, can act as the bearer of hereditary tendencies. This appears to be undoubtedly proved by the fact that the amount of material provided by the male parent for the development of an embryo is in almost all animals far smaller than the amount provided by the female parent.

In Mammalia the share contributed by the father probably only forms about one hundred-billionth part of that contributed by the mother, and yet nevertheless the influence of the former in heredity is on an average equal to that exerted by the latter²²⁵. Now, from the point of view of epigenesis, no molecule of the brain of an epileptic animal can reach the germ-cell except in a state of solution, and therefore no direct increase in the germ-plasm can be referred to such molecules, quite apart from the fact that such addition, even if possible, could not be of any value, because the last stage of the epileptic tendency must be represented in the nerve-cells and nerve-fibres of the diseased brain, while the first stage ought to be represented in the germ-cell.

It may be safely asserted that according to the theory of epigenesis the germ-cells cannot be influenced except as regards their nutrition. Nutritive changes may be imagined to occur through the varying trophic influence of the nervous system upon the sexual organs, but the structure of the germ-plasm cannot be altered by mere nutritive changes, or at all events it cannot be altered in that distinct and definite direction which is required by the supposed transmission of acquired epilepsy.

Thus the transmission of artificially produced epilepsy can neither be explained upon the epigenetic theory, nor upon the theory of preformation; it can only be rendered intelligible if we suppose that the appearance of the disease in the offspring depends upon the introduction and presence of living germs, viz. of microbes. The supposed transmission of this artificially produced disease is the only definite instance which has been hitherto brought forward in support of the transmission of acquired characters. I believe that I have shown that such support is deceptive, not because there is any uncertainty about the fact of the transmission itself, but because it is a transmission which cannot depend upon heredity, and is in all probability due to infection.

Ever since I began to doubt the transmission of acquired characters, I have been unable to meet with a single instance which could shake my conviction. There were many instances in which hereditary transmission was clearly established, but in none of them was there any reason to suppose that the characters transmitted were really acquired. For example, Fritz Müller has recently informed me of an instance in which he believes that there can be no doubt of the transmission of acquired characters. His observations are so interesting in several respects that I will quote them here. He says in his letter, ‘Among the bastards of two species of Abutilon, in which I had never observed hexamerous flowers, there was a single plant with a few such blossoms. As these flowers are sterile with the pollen of the same plant, I was obliged to fertilize it with pollen from another plant bearing only pentamerous flowers, in order to obtain seeds from the former. For three weeks I examined all the flowers from a plant grown from such seed, finding 145 pentamerous, 103 hexamerous, and 13 heptamerous flowers. I examined similarly the flowers of another plant produced from seed obtained from pentamerous flowers from the same parent plants. There were 454 pentamerous and 6 hexamerous flowers, and hence only 1·3 per cent. of the latter kind.’

It must certainly be admitted that the large proportion of abnormal hexamerous flowers depends upon heredity in the instance first quoted; but the hexamerous condition is not an acquired character; it is merely the first appearance of a new innate character. It is not due to the reaction of the vegetable organism under some external stimulus, for it appeared in a plant exposed to conditions similar to those which acted upon the other plant which only produced the normal pentamerous flowers. It must therefore have resulted from the tendencies which were present in the germ from which the plant itself developed, either as a spontaneous change in the germ-plasm or through the combination of two parental germ-plasms—a combination which may lead to the appearance or the reality of a new character. We know that the germ-plasm of each individual is not a simple substance, but possesses a very complex composition, for it consists of a number of ancestral germ-plasms represented in very different proportions. Now, although we cannot learn anything directly about the processes of growth of the germ-plasm, and its resulting ontogenetic stages, yet we do know, chiefly from observations upon man, that the characters of ancestors appear in the offspring in very different combinations and in very different degrees of strength. This may, perhaps, be explained by assuming that in the union of parental germ-plasms which takes place at fertilization, the contained ancestral germ-plasms unite in different ways, and thus come to grow with different strengths. Certain ancestral germ-plasms will meet and together produce a double effect: other opposed germ-plasms will neutralize each other; and between these two extremes all intermediate conditions will occur. And these combinations will not only take place at fertilization, but also at every stage of the whole ontogenetic history, for each stage is represented by its idioplasm, which is itself composed of ancestral idioplasms.

We do not yet know enough to be able to prove in detail the manner in which new characters may arise from such a combination of different kinds of germ-plasm. And yet it appears to me that such a view, e.g. in the case of the variation of buds, is by far the most natural. There is indeed a single example in which we can, to some extent, understand how it is that a new character may arise by these means. Certain canary-birds have a tuft of feathers on the head, but if two such birds are paired, their descendants are generally bare-headed, instead of having larger tufts²²⁶. The formation of a tuft depends upon the fact that the feathers are scanty and in fact absent from part of the skin of the head. Now when the scanty plumage of both parents is combined in the offspring the latter is bare-headed. Hence by the combination of ancestral characters a new character (bare-headedness) is produced, and one which is hardly likely to have ever occurred in the ancestors of existing canaries.

We do not know the causes which have been in operation when a flower possesses one petal more than the usual number, any more than we can explain why it is that one star-fish has five and another six rays. We cannot unravel the details of the mysterious relationship between two parent germ-plasms, each of which is composed of a countless number of ancestral germ-plasms from the first and second back to the nth degree. But we can nevertheless maintain in a general way that such irregularities are the result of this complex struggle between the germ-plasms in the ovum and the idioplasms in the subsequent stages of the developing organism, and that they are not the result of external influences.

If, however, acquired characters are brought forward in connexion with the question of the transformation of species, the term ‘acquired’ must only be applied to those characters which do not arise from within the organism, but which arise as the reaction of the organism under some external stimulus, most commonly as the consequence of the increased or diminished use of an organ or part. We have then to learn whether the altered conditions of life, by forcing an organism to adopt new habits, can by such means lead directly, and not indirectly through natural selection, to the transformation of the species; or whether the effects of increased or diminished use of certain parts, implied by the new habits, are restricted to the individual itself, and therefore powerless to effect any direct modification of the species.

Fritz Müller’s observation is also interesting in another respect: it appears to controvert my views upon heredity as expressed in the theory of the continuity of the germ-plasm. If a single flower can transmit to its descendants special peculiarities which were not possessed by its ancestors, we seem to be driven to the conclusion that the ancestral germ-plasm has not passed into the flower in question, but that new germ-plasm has been formed, inasmuch as the new characters are derived from the flower itself, and not from any of its ancestors. I think, however, that the observation admits of another interpretation: a specimen of Abutilon with many hundred flowers is not a single individual, but a colony consisting of numerous individuals which have arisen by budding from the first individual developed from the seed.

I have not hitherto considered budding in relation to my theories, but it is obvious that it is to be explained from my point of view, by supposing that the germ-plasm which passes on into a budding individual consists not only of the unchanged idioplasm of the first ontogenetic stage (germ-plasm), but of this substance altered, so far as to correspond with the altered structure of the individual which arises from it—viz. the rootless shoot which springs from the stem or branches. The alteration must be very slight, and perhaps quite insignificant, for it is possible that the differences between the secondary shoots and the primary plant may chiefly depend upon the changed conditions of development, which takes place beneath the earth in the latter case, and in the tissues of the plant in the former. Thus we may imagine that the idioplasm, when it developes into a flowering shoot, produces at the same time the germ-cells which are found in the latter. We thus approach an understanding of Fritz Müller’s observation; for if the whole shoot which produces the flower arises from the same idioplasm which also forms its germ-cells, we can readily understand why the latter should contain the same hereditary tendencies which were previously expressed in the flower which produced them. The fact that variations may occur in a single shoot depends upon the changes explained above, which occur in the idioplasm during the course of its growth, as a result of the varying proportions in which the ancestral idioplasms may be contained in it.

Fritz Müller’s observation affords a beautiful confirmation of this view, for if the flower itself transmitted the hexamerous condition to its germ-cells, we could not understand why some of the extremely rare hexamerous flowers were produced by the crossing of two pentamerous flowers, in the control experiment. An explanation of this fact can only be found in the assumption that the germ-plasm contained in the mother plant, during its growth and consequent distribution through all the branches of the colony, became arranged into a combination of idioplasms, which, whenever it predominated (as it did at certain places), necessarily led to the formation of hexamerous flowers. I will not consider here the question as to whether this combination is to be looked upon as an instance of reversion, or whether it represents something new. Such a question is of no importance for our present purpose; but the hexamerous flowers of the control experiment prove, in my opinion, that germ-plasm containing the requisite combination was distributed in the mother plant and also existed, but in insufficient amount, in shoots which did not produce any hexamerous flowers.

Appendix V. On the Origin of Parthenogenesis²²⁷

The transformation of heterogeny into pure parthenogenesis has obviously been produced by other causes as well as by those mentioned in the main part of this paper. Other and quite different circumstances have also had a share in its production. Pure parthenogenesis may be produced without the intermediate condition of heterogeny. Thus, for example, the pure and exclusive parthenogenesis with which the large Phyllopod crustacean, Apus, is reproduced at most of its habitats, has not arisen from the loss of previously existent sexual generations, but simply from the non-appearance of males, accompanied by the simultaneous acquisition of the power, on the part of the females, of producing eggs which do not require fertilization. This is proved by the fact that males occur in certain scattered colonies of this species, and sometimes they are even present in considerable numbers. But even if we were not aware of these facts, the same conclusions might nevertheless have been drawn from the fact that Apus produces eggs of only one form—viz. resting eggs with hard shells. In every case in which parthenogenesis has been first introduced in alternation with sexual reproduction, the resting eggs are produced by the latter generations, while the parthogenetic generations produce eggs with thin shells, in which the embryo developes and hatches very rapidly. In this way parthenogenesis leads to a rapid increase of the colony. In Apus such increase in the number of individuals is gained in an entirely different manner, viz. by the fact that all the animals become females, which produce eggs at a very early age, and continue producing them in increasing fertility for the whole of their life. In this manner an enormous number of eggs collects at the bottom of the pool inhabited by the colony, so that after it has dried up, in spite of loss from various destructive agencies, there will still remain a sufficiency of eggs to reproduce a numerous colony, as soon as the pool has filled again.

This form of parthenogenetic reproduction is especially well suited to the needs of species inhabiting small pools which entirely depend upon rain-fall, and which may disappear at any time. In these cases the time during which the colony can live is often too short to permit the production of several generations even from rapidly developing summer-eggs. Under these circumstances the pool would often suddenly dry up before the series of parthenogenetic generations had been run through, and hence before the appearance of the sexual generation and resting eggs. In all such cases the colony would be exterminated.

This consideration might lead us to think that Crustacea, such as the Daphnidae, which develope by means of heterogeny, would hardly be able to exist in small pools filled by the rain; but here also nature has met the difficulty by another adaptation. As I have shown in a previous paper²²⁸, the heterogeny of the species of Daphnidae which inhabit such pools is modified in such a manner, that only the first generation produced from the resting eggs consists of purely parthenogenetic females, while the second includes many sexual animals, so that resting eggs are produced and laid, and the continuance of the colony is secured a few days after it has been first founded; viz. after the appearance of the first generation.

But it is also certain that in the Daphnidae, heterogeny may pass into pure parthenogenesis by the non-appearance of the sexual generations. This seems to have taken place in certain species of Bosmina and Chydorus, although perhaps only in those colonies of which the continuance is secured for the whole year; viz. those which inhabit lakes, water-pipes, or wells in which the water cannot freeze. In certain insects also (e. g. Rhodites rosae) pure parthenogenesis seems to be produced in a similar manner, by the non-appearance of males.

But the utility which we may look upon as the cause of parthenogenesis is by no means so clear in all cases. Sometimes, especially in certain species of Ostracoda, its appearance seems almost like a mere caprice of nature. In this group of the Crustacea, one species may be purely parthenogenetic, while a second reproduces itself by the sexual method, and a third by an alternation of the two methods: and yet all these species may be very closely allied and may frequently live in the same locality and apparently with the same habit of life. But it must not be forgotten that it is only with the greatest difficulty that we can acquire knowledge about the details of the life of these minute forms, and that where we can only recognize the appearance of identical conditions, there may be highly important differences in nutrition, habits, enemies and the means by which they are resisted, and in the mode by which the prey is captured—circumstances which may place two species living in the same locality upon an entirely different basis of existence. It is not merely probable that this is the case; for the fact that certain species have modified their modes of reproduction is in itself a sufficient proof of the validity of the conclusions which have just been advanced.

The fact that different methods of reproduction may obtain in different colonies of the same species, although with thoroughly identical habits, may depend upon differences in the external conditions (as in Bosmina and Chydorus mentioned above), or upon the fact that the transition from sexual to parthenogenetic reproduction is not effected with the same ease and rapidity in all the colonies of the same species. As long as males continue to make their appearance in a colony of Apus, sexual reproduction cannot wholly disappear. Although we are unable to appreciate, with any degree of certainty, the causes by which sex is determined, we may nevertheless confidently maintain that such determining influences may be different in two widely separated colonies. As soon, however, as parthenogenesis becomes advantageous to the species, securing its existence more efficiently than sexual reproduction, it will not only be the case that the colonies which produce the fewest males will gain advantage, but within the limits of the colony itself, those females will gain an advantage which produce eggs that can develope without fertilization. When the males are only present in small numbers, it must be very uncertain whether any given female will be fertilized: if therefore the eggs of such a female required fertilization in order to develope, it is clear that there would be great danger of entire failure in this necessary condition. In other words:—as soon as any females begin to produce eggs which are capable of development without fertilization, from that very time a tendency towards the loss of sexual reproduction springs into existence. It seems, however, that the power of producing eggs which can develope without fertilization is very widely distributed among the Arthropoda.

Appendix VI. W. K. Brooks’ Theory of Heredity²²⁹

The only theory of heredity which, at any rate in one point, agrees with my own, was brought forward two years ago by W. K. Brooks of Baltimore²³⁰. The point of agreement lies in the fact that Brooks also looks upon sexual reproduction as the means employed by nature in order to produce variation. The manner in which he supposes that the variability arises is, however, very different from that suggested in my theory, and our fundamental conceptions are also widely divergent. While I look upon the continuity of the germ-plasm as the foundation of my theory of heredity, and therefore believe that permanent hereditary variability can only have arisen through some direct change in the germ-plasm effected by external influences, or following from the varied combinations which are due to the mixture of two individually distinct germ-plasms at each act of fertilization, Brooks, on the other hand, bases his theory upon the transmission of acquired characters, and upon the idea which I have previously called ‘the cyclical development of the germ-plasm.’

Brooks’ theory of heredity is a modification of Darwin’s pangenesis, for Brooks also assumes that minute gemmules are thrown off by each cell in the body of the higher organisms; but such gemmules are not emitted always, and under all circumstances, but only when the cell is subjected to unaccustomed conditions. During the persistence of the ordinary conditions to which it is adapted, the cell continues to perform its special functions as part of the body, but as soon as the conditions of life become unfavourable and its functions are disturbed, the cell ‘throws off minute particles which are its germs or gemmules.’

These gemmules may then pass into any part of the organism; they may penetrate the ova in the ovary, or may enter into a bud, but the male germ-cells possess a special power of attracting them and of storing them up within themselves.

According to Brooks, variability arises as a consequence of the fact that each gemmule of the sperm-cell unites, during fertilization, with that part of the ovum which, in the course of development, is destined to become a cell corresponding to that from which the gemmule has been derived.

Now, when this cell developes in the offspring, it must, as a hybrid, have a tendency to vary. The ova themselves, as cells, are subject to the same laws; and the cells of the organism will continue to vary until one of the variations is made use of by natural selection. As soon as this is the case, the organism becomes, ipso facto, adapted to its conditions; and the production of gemmules ceases, and with it the manifestation of variability itself, for the cells of the organism then derive the whole of their qualities from the egg, and being no longer hybrid, have no tendency to vary. For the same reason the ova themselves will also cease to vary, and the favourable variation will be transmitted from generation to generation in a stereotyped succession, until unfavourable conditions arise, and again lead to a fresh disposition to vary.

In this way Brooks²³¹ attempts to mediate between Darwin and Lamarck, for he assumes, on the one hand, that external influences render the body or one of its parts variable, while, on the other hand, the nature of the successful variations is determined by natural selection. There is, however, a difference between the views of Brooks and Darwin, although not a fundamental difference. Darwin also holds that the organism becomes variable by the operation of external influences, and he further assumes that changes acquired in this way can be communicated to the germ and transmitted to the offspring. But according to his hypothesis, every part of the organism is continually throwing off gemmules which may be collected in the germ-cells of the animal, while, according to Brooks, this only takes place in those parts which are placed under unfavourable conditions or the function of which is in some way disturbed. In this manner the ingenious author attempts to diminish the incredible number of gemmules which, according to Darwin’s theory, must collect in the germ-cells. At the same time he endeavours to show that those parts must always vary which are no longer well adapted to the conditions of life.

I am afraid, however, that Brooks is confounding two things which are in reality very different, and which ought necessarily to be treated separately if we wish to arrive at correct conclusions: viz., the adaptation of a part of the body to the body itself, and its adaptation to external conditions. The first of these adaptations may exist without the second. How can those parts become variable which are badly adapted to the external conditions, but are nevertheless in complete harmony with the other parts of the body? If the conditions of life of the cells which constitute the part in question must become unfavourable, in order that the gemmules which produce variation may be thrown off, it is obvious that such a result would not occur in the case mentioned above. Suppose, for example, that the spines of a hedgehog are not sufficiently long or sharply pointed to afford protection to the animal, how could such an unfavourable development afford the occasion for the throwing off of gemmules, and a resulting variability of the spines, inasmuch as the epidermic tissue in which these structures arise, remains under completely normal and favourable conditions, whatever length or sharpness the spines may attain? The conditions of the epidermis are not unfavourably affected because, as the result of short and blunt spines, the number of hedgehogs is reduced to far below the average. Or consider the case of a brown caterpillar which would gain great advantage by becoming green; what reason is there for believing that the cells of the skin are placed in unfavourable conditions, because, in consequence of the brown colour, far more caterpillars are detected by their enemies, than would have been the case if the colour were green? And the case is the same with all adaptations. Harmony between the parts of the organism is an essential condition for the existence of the individual. If it is wanting, the individual is doomed; but such harmony between any one part and all others, i. e. proper nutrition for each part, and adequate performance of its proper function, can never be disturbed by the fact that the part in question is insufficiently adapted to the outer conditions of life. According to Darwin, all the cells of the body are continually throwing off gemmules, and against such an assumption no similar objection can be raised. It can only be objected that the assumption has never been proved, and that it is extremely improbable.

A further essential difference between Darwin’s theory of pangenesis and Brooks’ hypothesis lies in the fact that Brooks holds that the male and female germ-cells play a different part, and that they tend to become charged with gemmules in different degrees, the egg-cell containing a far smaller number than the sperm-cell. According to Brooks the egg-cell is the conservative principle which brings about the permanent transmission of the true characters of the race or species, while he believes that the sperm-cell is the progressive principle which causes variation.

The transformation of species is therefore believed to take place, for the most part, as follows:—those parts which are placed in unfavourable conditions by the operation of external influences, and which have varied, throw off gemmules which reach the sperm-cells, and the latter by fertilization further propagate the variation. An increase of variation is produced because the gemmules which reach the egg through the sperm-cell may unite or conjugate with parts of the former which are not the exact equivalents of the cells from which the gemmules arose, but only very nearly related to them. Brooks calls this ‘hybridization,’ and he concludes that, just as hybrids are more variable than pure species, so such hybridized cells are also more variable than other cells.

The author has attempted to work out the details of his theory with great ingenuity, and as far as possible to support his assumptions by facts. Moreover, it cannot be denied that there are certain facts which seem to indicate that the male germ-cell plays a different part from that taken by the female germ-cell in the formation of a new organism.

For example, it is well known that the offspring of a horse and an ass is different when the male parent is a horse from what it is when the male parent is an ass. A stallion and a female ass produce a hinny which is more like a horse, while a male ass and a mare produce a mule which is said to be more like an ass²³². I will refrain from considering here the opinion of several authors (Darwin, Flourens, and Bechstein) that the influence of the ass is stronger in both cases, only predominating to a less extent when the ass is the female parent; and I will for the sake of brevity accept Brooks’ opinion that in these cases the influence of the father is greater than that of the mother. Were this so in all cross-breeding between different species and in all cases of normal fertilization, we should be compelled to conclude that the influences of the male and female germ-plasms upon the offspring differ at any rate in strength. But this is by no means always the case, for even in horses the reverse may occur. Thus it is stated that certain female race-horses have always transmitted their own peculiarities, while others allowed those of the stallion to preponderate.

In the human species the influence of the mother preponderates quite as often as that of the father, although in many families most of the children may take after either parent. There is nevertheless hardly any large family in which all the children take after the same parent. If we now try to explain the preponderating influence of one parent by the supposition of a greater strength in hereditary power, without first inquiring after some deeper cause, I think the only conclusion warranted by the facts before us is that this power is rarely or never equal in both of the conjugating germ-cells, but that even within the same species, sometimes the male and sometimes the female is the stronger, and that the strength may even vary in the different offspring of the same individuals, as we so frequently see in human families. The egg-cells of the same mother which ripen one after the other, and also the sperm-cells of the same father, must therefore present variations in the strength of their hereditary power. It is then hardly to be wondered at that the relative hereditary power of the germ-cells in different species should vary, although we cannot as yet understand why this should be the case.

It would not be very difficult to render these facts intelligible in a general way by an appeal to physiological principles. The quantity of germ-plasm contained in a germ-cell is very minute, and together with the idioplasms of the various ontogenetic stages to which it gives rise, it must be continually increased by assimilation during the development of the organism. If now this power of assimilation varied in intensity, a relatively rapid growth of the idioplasm derived from one of the parents would ensue, and with it the preponderance of the hereditary tendencies of the parent in question. Now, it is obvious that no two cells of the same kind are entirely identical, and hence there must be differences in their powers of assimilation. Thus the varying hereditary powers of the egg-cells produced from the same ovary become explicable, and still more easily the varying powers of the germ-cells produced in the ovaries or testes of different individuals of the same species; most easily of all the differences observable in this respect between the germ-cells of different species.