the female element of another; that is, we have true sexual reproduction in its simplest form.

Among the lower animals and most plants both sexes are united in the same individual, but the law of physiological division of labor, the principle that an organ or organism, like a machine, can do some one thing better and with less expenditure of force when it is specially adapted to this one thing than when it is generally adapted for several functions, would lead to the preservation by natural selection of any variations in the direction of a separation of the sexes, and we should therefore expect to find among the higher animals what we actually do find-the restriction of the male function to certain individuals, and the restriction of the female function to others. From this time forward the male is an organism specialized for the production of the variable element in the reproductive process, and the female an organism specialized for the production of the conservative element. We soon meet with structural peculiarities adapted to aid and perfect the performance of these respective functions; and the various organs, habits, and instincts by which, among the higher animals, the rearing of young is provided for, form one of the most interesting chapters of natural science. On a priori grounds we should expect a still greater specialization to make its appearance. Since the male organism has for its function the production of the variable reproductive element, and since variations which originate in a male have their perpetuation especially provided for, it would clearly be of advantage that the male organism should acquire a peculiar tendency to vary, and any steps in this direction would accordingly be seized upon by natural selection and perpetuated. The female organism, on the other hand, having for its function the

transmission of the established hereditary features of the species, we should expect the female to gradually acquire a tendency to develop these general characteristics more perfectly than the male. The male organism would thus gradually become the variable organism, as well as the transmitter of variations, and the female organism would become the conservative organism, as well as the originator of the conservative element in reproduction.

The study of the higher forms of life shows that this specialization has actually taken place in many cases, and that, in nearly all cases in which the sexes differ in peculiarities not actually concerned in reproduction, the male has varied more than the female. The amount of variation which any organism has lately undergone may be learned in two ways-by a comparison of allied species, and by a comparison of the adult with the young. In a genus which comprises several species the characteristics which these species have in common are due to heredity from a common ancestor, and are therefore older than features which are confined to any one species. Now, it is a well-known ornithological law that the females of allied species of birds are very much more alike than the males, and that in some cases where the females can hardly be distinguished the males are very conspicuously different-so much so that there is not the least danger of confounding them. Countless examples will present themselves to any one who is at all familiar with birds, and those who are not can at once find ample proof by glancing through any illustrated work on ornithology—Gould's "Humming-Birds," for example.

The greater variability of the male is also shown by a comparison of the adult male and female with the immature birds of both sexes. Since the growing animal tends to recapitulate, during its own development, the

changes through which its ancestors have passed, substantially in the order in which they first appeared, it follows that, in cases where the sexes are unlike, the one which is most different from the young is the one which has varied. Now, it is only necessary to compare the nearly full-grown young of our domestic fowls with the adult cock and hen, to perceive that the adult hen agrees with the young of both sexes in lacking such male characteristics as the highly ornamented tail-feathers, the brilliant plumage, the distended comb, the spurs, and the capacity to crow. Countless similar illustrations might be given to show the great tendency of the male to vary, but the above are sufficient for the purposes of our argument. As both sexes usually retain the more general specific and generic characteristics, and are alike as far as these are concerned, it is a little more difficult to show the conservative constitution of the female than it is to prove the male tendency to vary. Among the Barnacles there are a few species the males and females of which differ remarkably. The female is an ordinary barnacle, with all the peculiarities of the group fully developed, while the male is a small parasite upon the body of the female, and is so different from the female of its own species, and from all ordinary barnacles, that no one would ever recognize, in the adult male, any affinity whatever to its closest allies. All of the hereditary race characteristics are wanting: the limbs, digestive organs, and most of the muscles and nerves have disappeared, as they are not needed by a parasitic animal; and the male is little more than a reproductive organ attached to the body of the female. It is only when the development of the male is studied that we obtain any proof of its specific identity with the female. The young of both sexes are alike, and the developing

male shares with the female the characteristics which unite them to the other barnacles, and which are due to descent from a common form. The female keeps these hereditary characteristics through life, while the male soon loses them entirely.

These facts seem to be sufficient to prove that the specialization which we should expect to find among the higher animals with separate sexes does exist, and that the male organism is especially and peculiarly variable, and the female organism especially and peculiarly conservative.

Leaving this aspect of our subject for the present, let us look at it from a somewhat different point of view. The history of the evolution of life has not only an objective side, but something which may with perfect propriety be spoken of as a subjective aspect. The progress which is shown objectively as greater and greater specialization of structure, and a closer and closer adaptation of the organism to the conditions of the external world, has been well described by Herbert Spencer, as the increasing delicacy, exactness, and scope of the adjustment between internal and external relations. Seen in its subjective aspect, each of the steps in the growth of this adjustment is a recognition of a scientific law, the perception of the permanency of a relation between external phenomena; for science is simply the recognition of the order of nature.

When a Rhizopod discriminates between the contact of a large body and that of a small one, and draws in its pseudopodia and shrinks into as compact a shape as possible in order to escape the danger which the past experience of the race has shown to be related to the former sensation, or when it expands its pseudopodia in order to ingulf and digest the body which has caused

the second sensation, it furnishes proof that its scientific education has begun. Of course I do not intend to say that the order of nature, according to which the Rhizopod adjusts its actions, is consciously apprehended, but simply that it is the experience of the existence of this order which determines the action. Throughout the whole course of the evolution of one of the higher organisms each variation which served to bring about a closer harmony between the organism and its environment, and was accordingly preserved by natural selection, and added on to the series of hereditary structures and functions, was in its subjective aspect the experience of a new external connection, a new step in the recognition of natural law, an advance in scientific. knowledge. Human advancement is of course widely different from the slow progress of the lower forms of life, but it is fundamentally the same. Experience is continually spreading over new fields, and bringing about a more wide and exact recognition of the persistent relations of the external world. The scientific laws thus recognized then gradually take the shape of principles or laws of conduct, according to which actions are determined in those cases where experience has shown that they apply. Those laws of conduct which have been long recognized gradually assume the shape of habits or intuitions, according to which conduct is almost unconsciously regulated, and the habit finally becomes established as one of the hereditary characteris-, tics of the race.

We are apt to confine our attention to the subjective side of human advancement, and to neglect the structural side, and at the same time to neglect the subjective side of the evolution of the lower forms of life, and to confine our attention to the structural side, but of