THE ORIGIN OF SPECIES. 



57 



ready to start life as a frog, we should Lave some- 

 thing in a measure comparable with the state of the 

 chicken as just described. But it passes its fish stage 

 in a free state j and the changes which result in the 



Fig:. 13. — Outline of vestigial (p. 79) gills of an embryo dnck incubated for 

 five days. E, eye ; Br, I'ronc of brain ', C C, " visceral (or bronchial) clefts," 

 i.e. gill-spaces ; BB, parts which will afterwards ,ioin in the middle line and 

 give rise to the lower half of the beak. (Elnlarged.) 



obliteration of its gills and the modification of their 

 blood-vessels to suit the necessities of the adult stage 

 take place while it is swimming about. The larva 

 of the butterfly, like the larva of the frog, presents 

 the likeness of an adult animal of a lower type ; the 

 young frog is a fish, the young butterfly is a worm. 

 The startling changes which such animals undergo 

 become intelligible when we confront the hypothesis 

 that the different stages present the likeness of a 

 progressive series of ancestral forms. In the case of 

 the young, animal enclosed within the egg, there are 

 many other examples, besides the one already named. 



