Prof. Huxley says, "it is indeed a conceivable (?) supposition that every species of rhinoceros and every species of hyæna, in the long succession of forms between the Miocene and the present species, was separately constructed out of dust, or out of nothing, by supernatural power; but until I receive distinct evidence of the fact, I refuse to run the risk of insulting any sane man by supposing that he seriously holds such a notion."

It thus appears that the argument from "missing links," which to the general reader may appear so obviously fatal to the Darwinian theory, is to the student of palæontology by no means alarming. Our brief survey of the facts in the case has shown us first, that transitional varieties are always likely to have been less numerous in individuals than the well-defined species which they serve to connect; secondly, that the geologic eras which have left in the rocks the record of their organic life have been usually the eras in which variation and extinction have been least rapid, and in which accordingly transitional varieties have been least numerous; and thirdly, that in spite of all these adverse circumstances, transitional forms have already been discovered in considerable numbers, while it is fair to expect that many more will be discovered when by and by we have come to know the earth's surface more intimately.

Of all the objections which have been urged against the theory of natural selection, this objection, from the paucity of transitional forms, is the least weighty, though probably the most obvious. The second objection which we have to consider, though less immediately obvious, is more weighty; and though there is no reason for regarding it as insuperable, we must admit that it has not yet been entirely disposed of. This objection is implicated with the difference

perhaps near the point of departure of the anthropoid apes from the lower inonkeys and lemurs. See the anatomical evidence very well presented in Mr. Mivart's recent work on Man and Apes.

between the morphological and the physiological definitions of species, and is usually known as the argument from the infertility of hybrids. As ordinarily stated, indeed, this argument is merely the expression of a sorry confusion of ideas. By a curious misunderstanding the infertility of the mule is often urged as a direct objection to the Darwinian theory. But this is putting the cart before the horse. It is not the infertility of the offspring of the horse and the ass which should be cited as an obstacle to the theory of natural selection, but it is the fertility of the offspring of the carrierpigeon and the pouter, or of the pouter and tumbler. Morphologically the carrier, the pouter, and the tumbler may well be regarded as distinct species artificially developed from a common wild stock; but so long as mutual infertility is held to be the physiological test by which we are to distinguish between varieties and species, it may be argued that, in spite of their great morphological differences, the carrier and the tumbler are only varieties and not true species. And going a step farther, it may be argued that until the theory of natural selection has accounted for the rise of infertility between races descended from a common stock, it has not completely performed the task of reconciling deduction with observation.

Against the derivation theory in general, this objection has no weight whatever. That races originally fertile together should, after long subjection to different sets of circumstances, become infertile with one another, is à priori in the highest degree probable, when we reflect upon the extreme sensitiveness of the reproductive system to changes of habit in the organism as a whole. When we remember that "the constitution of many wild animals is so altered by confinement that they will not breed even with their own females," we need not be surprised that the leopard and the lion, which during many ages have had very different habits of life, will not breed with each other. Nor need we wonder

that the horse and the ass, with less important differences in general habit, have become partially infertile together, to such an extent that their offspring are hopelessly barren. Though the modus operandi of this change is as yet illunderstood, it is nevertheless a change quite in harmony with what we know concerning the intimate dependence of the reproductive system upon the rest of the organism. And let us not fail to note that it is the achievement of this change in the capacities of the reproductive system which completes the demarcation between two bifurcating species, and finally prevents the indefinite multiplication of intermediate varieties.

But while this objection has no weight as against the theory of derivation in general, it may fairly be urged that the failure to explain the origination of mutual infertility is, for the present at least, a shortcoming on the part of the theory of natural selection. After the conclusive arguments brought up in our ninth chapter, the derivation theory will no longer, in the present work, be regarded as on trial: that the higher forms of life are derived from lower forms, will be taken as proved. But whether the theory of natural selection has completely fulfilled its proposed task of explaining the mode in which such derivation has been brought about, is quite another question. And while admitting the full force of the considerations alleged by Mr. Darwin, in his admirable chapter on Hybridism, it seems to me that there is a gap at this point which further research will be required to fill. As Prof. Huxley reminds us, "it must not be for

1 I doubt if the hypothesis of natural selection, taken alone, will afford the solution of this problem. It seems more likely that such considerations will have to enter as are presented in Mr. Spencer's Principles of Biology, vol. 1. pp. 209-291. Concerning what may be called the "dynamics of heredity," we know as yet but little; but as far as speculation has already gone, Mr. Darwin's theory of pangenesis seems to me decidedly inferior to Mr. Spencer's theory of physiological units. I do not discuss these theories here, because it is not necessary for the general purposes of this work It may do no harm. however, to remind some of my readers that "pangenesis" is merely

gotten that the really important fact, so far as the inquiry into the origin of species goes, is that there are such things in nature as groups of animals and of plants, whose members are incapable of fertile union with those of other groups; and that there are such things as hybrids, which are absolutely sterile when crossed with other hybrids. For if such phenomena as these were exhibited by only two of those assemblages of living objects, to which the name of species . . . is given, it would have to be accounted for by any theory of the origin of species, and every theory which could not account for it would be, so far, imperfect." 1

We have now reached a point at which we may pause for a moment to contemplate the theory of natural selection in its logical aspect, and to mark its character as a scientific hypothesis. A moment's inspection will reveal the absurdity of the thoughtless remark-sometimes heard from theologians and penny-a-liners-that the Darwinian theory rests upon purely gratuitous assumptions and can never be submitted to verification. On the contrary, the theory of natural selection, when analyzed, will be found to consist of eleven propositions, of which nine are demonstrated truths, the tenth is a corollary from its nine predecessors, and the eleventh is a perfectly legitimate postulate. Let us enumerate these propositions:1. More organisms perish than survive;

2. No two individuals are exactly alike;

3. Individual peculiarities are transmissible to offspring; 4. Individuals whose peculiarities bring them into closest adaptation with their environment, are those which survive and transmit their peculiar organizations;

5. The survival of the fittest thus tends to maintain an equilibrium between organisms and their environments;

a subsidiary hypothesis, with the possible inadequacy of which Mr. Darwin's main theory is in no way concerned.

1 Huxley, Lay Sermons, p. 303.

6. But the environment of every group of organisms is steadily, though slowly, changing;

7. Every group of organisms must accordingly change in average character, under penalty of extinction;

8. Changes due to individual variation are complicated by the law that a change set up in any one part of a highly complex and coherent aggregate, like an organism, initiates changes in other parts;

9. They are further complicated by the law that structures are nourished in proportion to their use;

10. From the foregoing nine propositions, each one of which is indisputably true, it is an inevitable corollary that changes thus set up and complicated must eventually alter the specific character of any given group of organisms;

11. It is postulated that, since the first appearance of life upon the earth's surface, sufficient time has elapsed to have enabled such causes as the foregoing to produce all the specific heterogeneity now witnessed.

It seems to me that this summary fairly represents the logical character of the theory of natural selection. The theory is so strong that no scientific writer is disposed to deny that the process of natural selection has always gone on and must continue to go on. And the inference cannot be avoided that in due course of time the process must work specific variations. The only purely hypothetical portion of the theory is the assumption that past geologic time has been long enough to allow of the total process of evolution by such infinitesimal increments. But concerning this assumption, it is the clear verdict of logic, that if the theory is thoroughly substantiated in all its other portions, we have the right to claim as much time as is needful, provided we do not run counter to conclusions legitimately reached by astronomy, geology, or physics. Now concerning the age of the earth, neither astronomy, nor geology, nor physics, has as yet had anything conclusive to say; and it must be left for