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Tropical sheep with long coarse hair usually hare goat-like horns. Inherited baldness in man is ofteii accompanied by deficient teeth, and the renewal of the hair in old age by a renewal of the teeth. The famous hairy Burmese had deficient teeth, and both peculiarities were hereditary. A Spanish dancer, Julia Pastrana, had a full beara and a double set of teeth, and the daily papers have recently contained an account of a man, living near Lebanon, Pennsylvania, with no hair, teeth, or sweat glands.
The homologous parts of plants often vary in the same way, as is well shown by certain compound flowers, in which the stamens and pistils closely resemble petals.
According to our view of the cause of variation we can easily see how gemmules from a cell in one hand might hybridize, and thus cause variation in the corresponding cells of all four extremities, or perhaps in the embryonic cell from which all these cells are derived, for in the same way that an animal can unite sexually either with another of its own race or with one which is somewhat less closely related to it, so I assume that a gemmule may unite with the particle of the ovum which corresponds to it, or with some other closely related particle. For example, agemmule which is thrown off from a particular epithelial cell may simply cause modification in the corresponding cell of the offspring, or it may cause modification in a cell which is to produce this particular cell and a number of others.
If each variation is purely fortuitous the number of generations which would be necessary in order to convert a species with black hair into a spacies with every hair brown or with every hair red is almost inconceivable, but this difficulty entirely disappears as soon as we recognize that gemmules from one part of the parent may affect all the homologous parts of the offspring in the same way and at the same time.
Males more Variable than Females.
One of the most remarkable and suggestive of the laws of variation is that in all the higher animals a part which is confined to males, or is more developed or of more functional importance in males than it is in females, is very much more variable than a part which is confined to females or is more important in females than it is in males.
The evidence for this remarkable law will be presented at length in Chapters VIII. and IX. The existence of such a law is absolutely inexplicable without the theory of heredity, but it is exactly what this theory would lead us to expect, for an organ which is most important in one sex is most likely to be influenced in this sex by changed conditions, and is therefore more likely to form gemmules in the body of the sex where it is most important than in the body of the opposite sex. An organ which is most important in males will therefore be most prolific of gemmules in males, while an organ which is most important in females will be most prolific of gemmules in females. Gemmules which are formed in the male body are vastly more likely to be transmitted to descendants than those which are formed in the female body. It follows that an organ which is most developed or most important in males must be vastly more likely to transmit gemmules to descendants, and therefore to vary in successive generations than an organ which is most developed or most important in females.
Another law which follows from the one which has just been stated is that males are as a rule more variable than females. This law has been noticed by Darwin and others, but no explanation has ever been advanced.
Summary of Last Two Chapters.
The study of hybrids and of variation has led to the discovery of a great number of general laws, all of which are perfectly explicable by the theory of heredity, and are precisely what it would lead us to look for, although most of them are absolutely inexplicable without it, and have no place in any other hypothesis which has ever been proposed to account for the phenomena of heredity.
The study of hybrids gives us a means of analyzing to a certain extent the influence of each sex in heredity, but our experiments in this direction are limited by the fact that organisms must be very closely related in order to breed together, and parents which are very closely related must be essentially alike in everything except the most recently acquired modifications. So far as they enable us to analyze the influences of the sexes, the results furnished by hybrids agree with the demands of our theory. This furnishes an explanation of the great variability of hybrids, as compared with the pure parents, and it also enables us to understand why hybrids from domestic races should be more variable than those from wild races.
The remarkable fact that the descendants of hybrids are more variable than the hybrids themselves receives a simple explanation by our assumption that exposure of the various cells of the body to unnatural conditions is the prime cause of variability, and that it acts indirectly by causing the production of gemmules.
Some of the recorded facts regarding hybrids are so very peculiar that it would be difficult to devise better tests than they furnish of the truth of our theory. What could be more curious or more opposed to the view that the sexes play similar parts in heredity than the fact that the offspring of a male hybrid and the female of a pure species is much more variable than the offspring of a female hybrid by a father of pure blood? Darwin's pangenesis hypothesis furnishes no explanation of this most remarkable fact, and none of the hypotheses of heredity which have been proposed from time to time are sufficiently definite to have any bearing upon a concrete case like this, but our theory that changed conditions of life cause a production of gemmules,.and that these are stored up in and transmitted by the male element, fits this case exactly. . .
The curious phenomena of reciprocal crosses, again, are just what our theory would lead us to expect, and it also furnishes us with an explanation of the fact that crossing so frequently causes reversion.
A comparison of sexual with asexual reproduction also gives us a means of analyzing the influences of the two sexual elements, for asexual reproduction is essentially reproduction with the male element left out, and the result of this omission is, as we should expect, the reduction of the tendency to vary to a minimum. At the same time that our theory explains the great rarity of bud variations, it admits of their occasional appearance, and it gives an explanation of the singular fact that bud variation is much less rare in plants which have long been cultivated than it is in wild forms.
The most remarkable of the laws of variation is the well-known law that changed conditions do not directly produce variation, but cause subsequent generations to vary. As changed conditions do not in themselves cause hereditary modification, but simply lead to the production of gemmules, we see why their effect should be manifested in succeeding generations, and we also see why variation is itself hereditary, for the variation of any particular cell will cause adjacent or related cells to throw off gemmules, and thus to produce variation in successive generations.
We can also understand why specific characters should he more variable than generic characters; why the species of large genera should vary more than the species of small genera; why a part developed in an unusual way or to an unusual degree should show a marked tendency to vary, and why secondary sexual characters should exhibit a similar tendency.
Unless our theory is true, what possible reason can there be why a part which is excessively developed in males should vary more than a part which is similarly developed in females alone, or why the males of our higher domesticated animals should be more variable than the females? Its power to deal with and interpret special cases of this kind separates our theory from all other attempts to explain the phenomena, and seems to show that there can be but one choice between it and any other explanation which has ever been proposed.
If we accept Darwin's view that variations are purely fortuitous, there are certain grave difficulties which must prevent us from giving the theory of natural selection unqualified acceptance as an adequate and complete explanation of the origin of species.
Natural selection can rarely lead to permanent modification unless many individuals tend to vary in nearly the same way at about the same time, and if variation is fortuitous the chance against this is very great indeed. While there is no reason to doubt that natural selection might bring about .all the changes which have led to the formation of a complicated organ, by the preservation of fortuitous variations, if time enough were given, there is