96 ORGANIC EVOLUTION 



organs : thus in time the eyes will become mere vestiges of 

 their former selves. Weismann's theory of germinal selec- 

 tion also may apply here. 1 



The great variety of forms among animals and plants, 

 their different degrees of complexity, the phenomena of 

 homology and of vestigial structures, are readily explained 

 by the theory of evolution, though without the aid of this 

 theory they are apparently meaningless to us. 



The phenomena of embryology as related to the theory of 

 evolution. 



In the study of the anatomy of different plants and 

 animals we find, as already stated, that they are of very 

 different degrees of complexity. We judge in general that 

 the simpler species are the more primitive and that the more 

 elaborate have been evolved from simpler forms, perhaps from 

 forms more or less like some we find living to-day. The 

 study of embryology gives us additional evidence of the truth 

 of this conclusion. We find that complexly organized ani- 

 mals and plants arise each from a single cell, the fertilized 

 egg, and gradually acquire new organs and a more compli- 

 cated structure, till finally the adult condition is reached 

 (Fig. 19). The series of stages of increasing complexity, 

 seen in the development of one of these higher forms, reminds 

 us of the taxonomic series in our classification of plants and 

 animals, in which we found all gradations in complexity from 



1 This theory of germinal selection can hardly be stated briefly and simply. 

 Weismann's own statement can be found in his essay "On Germinal Selection as a 

 Source of Definite Variation,' 1 published by The Open Court Publishing Company. 

 It will not be easily understood by one who has not followed Weismann's work. 



