THE ARGUMENT FROM EMBRYOLOGY 99 



eggs, not in water, but on the leaves of plants. 

 These eggs are larger than those of the common 

 frog— i.e., contain more" food-yolk — and the young 

 embryo is thereby enabled, just like the lobster or 

 crayfish, to develop to a later stage before hatching : 

 it passes through the tadpole stage within the egg. 

 and hatches, like the crayfish, in the form of its 

 parent. Although it passes through a gill-cleft stage 

 no gills are developed ; being of no use to the 

 embryo, it would be a sheer waste of time to form 

 them, and so they have dropped out of the ontogeny 

 or individual development. (Figs. 15, 16.) 



Exactly the same thing has happened in reptiles, 

 birds, and mammals, in which gill-clefts are found, 

 but gills are not developed. A similar tendency to 

 the omission or blotting out of useless characters is 

 seen in the development of all forms which have 

 sufficient food-yolk to carry them over the stages 

 at which these characters would be of functional 

 value before the time of hatching. 



In the case of embryos developed from small eggs 

 there is a tendency to distortion of the ancestral history 

 of a very different nature. Such embryos, owing to 

 the small supply of food-yolk in the egg, have to 

 hatch very quickly — i.e., not merely of small size, but 

 in a condition representing a very remote ancestral 

 stage. The intervening stages between the early 

 condition and the adult one have to be repeated 

 while the larva is enjoying a free existence : this 

 process will necessarily be slow, for the larva has not 

 merely to develop, but has to obtain for itself food, 

 at the expense of which the further development 



