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In tracing the evolution of the various organs we shall follow the method that has hitherto guided us, except that we shall now have to consider the ontogeny and phylogeny of the organs together.
But these hairs, like the hairs of plants, are thread-like appendages of the surface, and differ entirely from the hairs of the mammals in the details of their structure and development. The embryology of the hairs is known in all its details, but there are two different views as to their phylogeny.
Comparative anatomy shows us to-day, in the series of the double-nosed Vertebrates, from the fishes up to man, all the different stages in the development of the nose, which the advanced olfactory organ of the higher mammals has passed through at various periods in the course of its phylogeny.
When such a break is reached, the course of phylogeny is like picking up an interrupted trail, with the additional complication that the one we find is never quite like the one we left, and it is in such conditions that the systematist must apply his knowledge of the general progressive tendencies through the ages of change, to the determination of the particular changes he should expect to find in the special case before him, and so be enabled to recognize the footprints he is in search of.
Little by little I became familiar with the principles of embryonic evolution, ontogeny, and of biological evolution, phylogeny; realized, for the first time, my own history and that of the ancestors from whom I had developed and descended. I, this marvellously complicated being, torn by desires and despairs, was the result of the union of two microscopic cells.
Better some risk of gross thoughts and even acts, to which phylogeny and recapitulation so strongly incline him, than this subtle eviration. But if the boy is unduly repelled from the sphere of girls' interests, the girl is in some danger of being unduly drawn to his, and, as we saw above, of forgetting some of the ideals of her own sex.
We have previously seen how this very rudimentary beginning of the skull in man is formed ontogenetically from the "head-plates," and thus the fore end of the chorda is enclosed in the base of the skull. The phylogeny of the skull has made great progress during the last three decades through the joint attainments of comparative anatomy, ontogeny, and paleontology.
Bare descriptions cannot and should not inspire interest, whereas the driest anatomical facts, if seen in their broader relationships, at once assume a significance in the student's mind which may be attained in no other way. The first chapter is a brief statement of phylogeny, followed, as are succeeding chapters, by directions to the student regarding means of study.
And even if we wish to avoid the error of Haeckel and others who find a necessary connection between ontogeny and phylogeny, nevertheless the analogy will still entitle us to picture to ourselves the development of the whole range of living organisms. Such a representation will, of course, have only a subjective value.
A similar argument is used in regard to “ontogeny recapitulating phylogeny.” Palæontology does not disclose in the plant-world any “synthetic types,” which might have been the common primitive stock from which many now divergent branches have sprung, nor does it disclose any “transition links” really intermediate, for instance, between cryptogams and gymnosperms, or between gymnosperms and angiosperms.
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