60 DOCTRINE OF EVOLUTION 



brates led to the conclusion that the latter had evolved 

 from antecedents like the former, and had thus followed 

 them upon the earth ; now that sequence seems to have 

 some connection with the method by which a tadpole, 

 obviously not a fish but nevertheless actually fishlike, 

 changes into a frog, a member of a higher class of verte- 

 brates. This method is employed by developing frogs 

 apparently because it follows the ancestral order of 

 events, and because, so to speak, the only way a frog 

 knows how to become a frog is to develop from an egg 

 first into a fishlike tadpole and then to alter itself as 

 its ancestors did during their evolution in the past. 

 We begin to see, then, that in addition to the impressive 

 fact of development itself, the mode of organic trans- 

 formation is far more conclusive evidence of evolution, 

 because it reveals an order of events which' parallels 

 the order estabhshed by comparative anatomy as the 

 evolutionary sequence. 



However it is well to review some of the changes 

 by which a chick comes into existence before attempt- 

 ing to comprehend fully the fundamental principle 

 of development that the tadpole's history discloses to 

 us. The egg of a common fowl is certainly not a chick. 

 Within the calcareous shell are two delicate membranes 

 that enclose the white or albumen ; within this, swung 

 by two thickened cords of the albumen, is the yellow 

 yolk ball enclosed by a proper membrane of its own. 

 In the earliest condition, even before the albumen and 

 the shell are added and before the egg is laid, on one 

 side of the yolk-mass there is a tiny protoplasmic spot 

 which is at first a single cell and nothing more. The 

 hen's egg is relatively enormous, but nevertheless, like 



