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reason to doubt whether life has existed long enough to permit the evolution of existing forms in this way, and natural selection gives no account of the sudden appearance of considerable modifications, although the history of domestic animals shows us that such saltations do sometimes occur.
On the one hand we find that Darwin's assumption that variations are fortuitous involves us in grave difficulties, but on the other hand we find scarcely any evidence to show that permanent hereditary race modifications are ever directly produced by the action of external conditions, while we do find evidence for the opinion that race modifications are, as a rule, not due to this direct action, but to congenital variation.
Our theory furnishes an explanation which lies midway between Darwin's view of the origin of variation and the Lamarkian view, and thus enables us to escape both these difficulties, for it shows us how the influence of changed conditions upon an organism may give rise to congenital variation in later generations, and it also shows us why variations tend to appear at the time and place where they are needed. It also shows how a considerable modification may appear suddenly and become hereditary.
The correlated variation of homologous organs and the correlated modification of the various parts of a complicated organ are accepted by Darwin without explanation, but the theory of heredity shows us that these phenomena, the chance against the fortuitous occurrence of which is almost infinite, are due to the working of a very simple law.
When we review the ground and see how all the phenomena of hybridization and variation fall into their proper places; how the same simple explanation fits the most anomalous and exceptional phenomena as well as the more ordinary and simple cases, I think we must acknowledge that our theory is at least an approximation to the truth.
If it leads us to the discovery of truth, and thus ultimately contributes to the establishment of an explanation of the phenomena of heredity, its final acceptance in its present form is a matter of little moment. That it is a great advance beyond all the attempts which have been recorded seems obvious, and an examination of the ground which it covers certainly seems to show that it is a step in the right direction.
THE EVIDENCE FROM SECONDARY SEXUAL CHARACTERS.
The Nature of this Sort of Evidence. I HAVE already given many reasons for believing that the male reproductive organ is especially adapted for gathering up the gemmules which are thrown off by the cells of the body; and for transmitting them to the next generation by impregnation, thus giving rise to variation; while the transmission of the gemmules which are formed in the body of the female is not thus provided for.
If this supposition is correct, we should expect to find that a variation which first appears in a male should have more tendency to become hereditary than one which first appears in a female. Any slight change in either the male or the female body will, as we have already seen, cause all the cells which are either directly or indirectly influenced by the change to throw off gemmules. This will happen in a female body as well as in a male body, but the gemmules are, in the latter case, much more likely to be transmitted to descendants, and thus to give rise to more extended modification.
We should also expect to find that an organ which is confined to males is much more likely than one which is confined to females to undergo hereditary changes, for even if the parts of the female body give rise to gemmules as frequently as the parts of the male body, the chance of transmission is much less.
We should also expect to find that parts which are confined to males are more variable than parts confined to females; for variation in any part is due to inheritance of a gemmule from the corresponding part of one parent or the other, but when the part is found in only one parent the gemmule must come from that parent.
As transmission of gemmules by the mother is more rare than transmission by the father, it is plain that parts which are confined to the male should be expected to vary more than parts found in the female alone.
Finally we should expect the male body as a whole to be more variable than the female body, for the same reason.
In most cases it is impossible to trace any particular variation back to its first appearance. This is almost out of the question with wild animals, and most domesticated races have been formed so slowly that it is impossible to say whether the successive steps appeared in males or in females, nor can we be sure that a variation is new when it first attracts attention. Still it is interesting to note that the sudden variation which resulted in the ancon breed of sheep was first noticed in a male, although it is, of course, impossible to say whether it was due to inheritance of gemmules from the father rather than from the mother. Certain hereditary diseases and montrosities, such as albinism or polydactylism, are fully as often traceable to a male origin as they are to a female origin, but as we know that peculiarities of this kind frequently skip a generation or two, we can never be sure that we have traced them to their origin.
In the secondary sexual characters of animals we have a class of phenomena which are not rare and exceptional, for they are numbered by hundreds of thousands, and they can be observed and studied by every one.
A secondary sexual character is a peculiarity which is not directly concerned in the reproductive process, although it is normally either confined to one sex, or else is much more developed in one sex than it is in the other. The presence of a beard is a secondary sexual character of man; the comb, wattles, spurs and brilliant plumage of the domestic cock, the horns of a stag, the tusks of an elephant, the mane of a lion, or the brilliant plumage of the peacock or of the drake, are all of them examples of this sort of organs, for they are either confined to one sex, or else they are much more conspicuous and important in one sex than they are in the other.
They furnish, like hybrids, a means of disentangling or analyzing to some extent the influence of the two sexes in heredity, and I hope to show in this and the following chapters that they furnish evidence to prove
1. That in most animals with separate sexes the males of allied species differ more than the females from the ancestral type.
2. That organs which are confined to males or are of more importance or are more perfectly developed in them than in the females, are much more likely to give rise to hereditary modifications than parts which are confined to or are most developed in females.
3. That a part which is confined to or is most developed in males is more likely than a similar female part to vary.
4. That males are, as a rule, more variable than females.
5. That the male leads and the female follows in the evolution of new races.
There are two criteria which are of great use in the attempt to trace the path which a species has followed in its evolution. One of these is by comparison of a