D EG EN ERA TIOX. 5 3 



tadpole. But such recapitulative development is by 

 no means the rule. Quite arbitrarily, we find, it 

 is exhibited in one animal and not in a nearly 

 allied kind. Thus very many animals belonging 

 to the Ascidian group have no tadpole young — 

 just as some tree-frogs have no tadpoles. It is 

 quite possible, and often, more often than not, 

 occurs, that the vwst important part of the recapitu- 

 lative phases are absent from the developmental 

 history of an animal. The ^gg proceeds very rapidly 

 to produce the adult form, and all the wonderful 

 series of changes showing the animal's ancestry are 

 absolutely and completely omitted ; that is to say, 

 all those stages which are of importance for our 

 present purpose. Just as certain bodies pass from 

 the solid to the liquid state at a bound, omitting all 

 intermediate phases of consistence, but giving evidence 

 of " internal work " by the suggestive phenomenon of 

 latent heat — so do these embryos skip long tracts in 

 the historically continuous phases of form, and present 

 to us onlv the intanc^ible correlative ''internal work" 

 in place of the tangible series of embryonic changes 

 of shape. 



Now I want to put this case — a supposition — before 



