1902.] NATURAL SCIEN'CES OF PHILADELPHIA. 199 



of reproductive activity the orgauism may be less differeQtiated 

 structurally than at preceding stages, as in some extreme cases of 

 endoparasitism. Where alternation of generations (metagenesis) 

 occurs, there may be, as in the case of certain Hydrozoa, two 

 generations, each reproductively active, and the individuals of 

 which are structurally different : which of these is the perfected 

 stage ? And finally, when it is recalled that for certain organisms 

 there is a succession of stages of senility, it will be seen how diffi- 

 cult it is to define Avhat is meant by maturity or perfection of 

 organization. 



We have just seen that the period of reproductive activity does 

 not always fix sharply the stage of greatest perfection of the organ- 

 ism. This stage, from the phyletic standpoint, is that when the 

 organism has attained the end of the period which precedes senile 

 modification. This may be the stage of greatest structural differen- 

 tiation of the organism, or it may not. In the case of an extreme 

 parasite, such as certain of the highly degenerated Cii-ripedes and 

 Copepods, the larval stage may be the most perfect, from the stand- 

 point of general structure, for then the locomotory appendages, 

 sense organs and separateness of the nietameres are clearly pro- 

 nounced, while they degenerate later. But, if such a degenerate 

 parasite were classified at its earlier, non -parasitic, more structurally 

 perfect stage, such a classification would not represent the degree in 

 which its evolution has diverged from that of its relatives, and 

 would therefore rank the organism in a more primitive stage than 

 it deserves. It is the end stage w^hich shows how far the individual 

 has proceeded in its evolution, whether progressive or regressive, 

 while classification at an earlier stage, even though then the organ- 

 ism be structurally more complex, would not show the whole path 

 of descent. Maturity, from the phyletic standpoint, is the stage 

 preceding senility. It is not my purpose here to define senility, or 

 what characterizes it, and indeed, senility may appear in one part 

 before another has reached its full development, and at its first 

 appearance is difficult to detect. But we would say that the organ- 

 ism as a whole is not senile so long as it continues the power of 

 reproduction; and if there are several periods of reproduction, we 

 should not consider the organism to be senile as a whole until after 

 the last period. Thus, in the cases of birds which show successive 

 nuptial plumages and structures, or of mannnals which change 



