136 The Science of Life. 



truism that these develop because of immediately oper- 

 ative growth-conditions, or reactions between inherited 

 organization and environmental stimulus; but the whole 

 story becomes more luminous to us if we are otherwise 

 assured that the race of frogs sprang from a fish 

 ancestry. (d) It is said that increased precision of 

 embryological work discloses individual characteristics 

 at a very early stage in ontogeny, that even a blind 

 man could distinguish embryos of duck from those of 

 the fowl as early as the second or third day of incuba- 

 tion. Yet this does not seem to be inconsistent with a 

 general recapitulation. 



All are agreed that there is no completeness of re- 

 capitulation, else phylogeny would be a simpler business 

 than it is. As Haeckel, Balfour, and others have said, 

 ancestral stages may be dropped out in embryonic 

 development, or disguised by newer adaptive characters 

 in larval development. But in the dropping out there 

 must be some law. Why do certain ancestral charac- 

 ters recur, or apparently recur, while of others there is 

 no trace? Why does an embryo snake show gill-clefts 

 but no trace of fore-limbs? To this question Balfour 

 answered, "It is very possible that rudiments of the 

 branchial arches and clefts have been preserved be- 

 cause these structures were functional in the larva 

 (Amphibia) after they ceased to have any importance 

 in the adult; and that the limbs have disappeared even 

 in the embryo, because in the course of their gradual 

 atrophy there was no advantage to the organism in 

 their being preserved at any period of life ". 



Similarly, Prof. Sedgwick has maintained that when 

 there is a recapitulation of ancestral stages in embryonic 

 development, this implies that the characters in question 

 were retained as useful larval characters for a long time 

 after they had ceased to be directly functional in the 

 adult. 



Another evolutionary idea which has arisen out of 

 embryology is that of "the substitution of organs", 

 Substitution suggested by Nicolaus Kleinenberg (1842- 

 of Organs. 1897), one of Hseckel's numerous disciples, 

 and professor of zoology at Messina and Palermo. He 



