4 FREE AND FIXED ANIMALS 



in general can be regarded with greater propriety 

 as the vehicles of future adult types than as 

 recapitulations 1 of past ancestors. The larva, 

 looked at synthetically in its entirety as an 

 independent organism, apart from structural 

 details, is essentially the vehicle of the adult, 

 not that of an ancestral form. Thus the frog 

 doubtless had a fish-like ancestor, but the 

 tadpole, which is the fish-like larva of the frog, 

 does not recapitulate the fish-like ancestor of 

 the frog. The external facies of the tadpole is 

 common to the larval or postlarval stages of 

 many fishes, and represents a fundamental form 

 likely to recur by convergence. 



The oft-quoted Pilidium larva of Nemertine 

 worms is an actual vehicle within human ex- 

 perience, a vehicle from which the worm alights 

 when it has reached a certain stage on its 

 journey through life. The Tornaria larva of 

 Enteropneusta and the larval forms of Echino- 

 derms which are related to it are also obviously 

 nothing else but larval, i.e., not ancestral forms 

 in themselves. We may therefore repeat our 

 dictum that the nature of larval forms is that 

 of vehicles of the future rather than relics of the 

 past, true larval characters never having been 

 adult characters. 



1 For a recent discussion of the theory of recapitulation see the 

 following: Adam Sedgvvick, "The Influence of Darwin on the 

 Study of Animal Embryology" in "Darwin and Modern Science," 

 Cambridge, 1909. 



