THE REPRODUCTIVE FACTOR IN EVOLUTION, 



287 



sought rather to be physiologists, studying and interpreting the high- 

 est and intensest activity of things living. From the study of indi- 

 vidual structure they were wont to pass, indeed, to that of reproductive 

 structures, and thence even functions; hence, too, the pair and the 

 totality of the species did at length come successively into view; but 

 this with the individualistic theory of natural selection bulking as 

 practically all-important in the foreground, to which even sexual selec- 

 tion was a mere harmonious corollary. For us, however, this 

 perspective has become entirely reversed. The individual is a mere 

 link in the species, and its reproductive processes are thus of funda- 

 mental importance to the interpretation even of its self-maintaining 

 ones. Hence we no longer regard, with Darwin and the majority of 

 our brother naturalists, the operation of natural selection upon indi- 

 vidual characters as the simplest of problems, looking for residual 



-^SBpHsegise 



Fig. 104. — An Opossum (Didelphys dorsigera carrying its young on its back.- 



From Carus Sterne. 



explanation to sexual selection, and only in extreme difficulty invoking 

 the aid of " principles of correlation," " laws of growth," and the like, 

 viewed as almost inscrutably mysterious. On the contrary, it is the con- 

 tinual correlation, yet antithesis — the action and reaction — of vegetative 

 and reproductive processes in alternate preponderance, which seems to 

 us of fundamental importance, since to this the general rhythm of indi- 

 vidual and racial life runs fully parallel. Hence it is that we have the 

 primeval lily developing on the one hand the ideally vegetative grass, 

 yet also the supremely specialized reproductive orchid; and that we 

 can trace (as we hold) the same swing of divergent evolution, of 

 definite variation, in every natural order, nay, in every genus, often 

 even in the very varities of a species. Hence, too, it is that the rhythm 

 of hydroid and medusoid in the individual life of the typical forms 



