Page:Popular Science Monthly Volume 22.djvu/380

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that one is like the other with additions, but we find that the young are alike up to a certain point, where the divergence of the adults begins. If the letter A, in diagram 2, represents an adult animal, and the series of dots in the vertical line a few of the stages of its development between birth and maturity, the relationship between A and a closely allied species, B, would not usually be of such a kind that it PSM V22 D380 Schematic of evolutionary divergence between the young and mature.jpgFig. 2. could be expressed by the line A B, P but by a line B c, running back to a younger stage of A. Since the duration of mature life is usually much longer than the period of development, adults are generally more numerous than young individuals, and, as their hard parts are in most cases more fully developed, there is a much greater chance of finding adult than young specimens of fossil species. Even if we should find in a recent geological formation the adult fossil ancestor A of a recent species B, the agreement between the two would be inexact, and could not be fully perceived until we had compared a number of stages in the development of B with corresponding stages in the development of A; and a reference to diagram 2 will show that the actual relationship of B to still another species, D, might be just as close as its relationship to A, although B might be much more different from D than from A. The complexity of the case would be still further increased if D were met with in a more recent stratum than that which yielded A, and in this case the preservation, discovery, and identification of three distinct sets of young forms would be necessary before the relationships of A, B, and D could be unraveled. In nature few cases are so simple as this, and, where a dozen species are involved, the complexity would be so great that no one who has had any practical acquaintance with the difficulty of identifying immature animals with certainty, without rearing them, could have any hope of complete, exact, and definite evidence of phytogeny from fossils.

While this is true in every case, its truth is most obvious where animals have become adapted to new conditions of life, not by the acquisition of new specializations of structure, but by the loss of old ones. The occurrence of unquestionable cases of simplification, or what is usually called degradation, is well known to naturalists, but, as these cases are not so well known to the unscientific public as their signifi-