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THE POPULAR SCIENCE MONTHLY. 



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 be- 

 gins. 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 devel- 

 opment between birth and maturity, the relationship between A and a 

 closely allied species, B, would not usually be of such a kind that it 



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 de- 

 velopment, 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 fos- 

 sil 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 correspond- 

 ing stages in the development of 

 A ; and a reference to diagram 2 

 will show that the actual relation- 

 ship of B to still another species, D, might be just as close as its re- 

 lationship to A, although B might be much more different from D 

 than from A. The complexity of the case would be still further in- 

 creased if D were met with in a more recent stratum than that which 

 yielded A, and in this case the preservation, discovery, and identifica- 

 tion 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, ex- 

 act, 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 ac- 

 quisition 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- 



Fig. 2. 



