THE RECAPITULATION THEOEY. 25 



mating towards the fully formed- state,' and each stage bringing- 

 the animal, not merely as a whole, but as regards each of its 

 organs and parts, one step nearer to the perfect form. 



In the great majority of animals, however, the course of 

 development is not so straightforward. Even in Amphiosus 

 there are features in the early embryonic stages, such as the 

 communication between the neural tube and the digestive 

 cavity, which completely disappear during development, and 

 which have no relation to the adult condition of the animal. 



In the higher Vertebrates, far more striking instances occur. 

 In the embryo of a chick or of a mammal the structure and 

 relations of the heart and blood-vessels are for a time those of a 

 fish ; and for the attainment of the adult condition it is neces- 

 sary, not merely that new structures should -appear and new 

 relations be acquired, but that parts once present should actually 

 become obliterated. The frog, again, commences its free exist- 

 ence as a tadpole, which is really a fish, not merely as regards 

 its breathing organs, but in all details of its organisation ; and 

 the change from, the tadpole to the frog involves great modifica- 

 tion in the shape, size, and relations of almost all its organs, 

 with complete obliteration of parts such as gills and tail, which 

 were essential to the tadpole but are absent from the frog. 



It is to cases such as the frog, or as the butterfly, in which 

 the transition from larva to adult is even more extensive and 

 more abrupt, that the term metamorphosis is applied ; cases in 

 which the animal, instead of developing straight towards the 

 adult condition, in place of aiming straight at its goal, deviates 

 from the direct path, spends time and energy in developing and 

 elaborating organs which, though in perfect keeping with its 

 actual mode of existence, yet have no relation to the state it is 

 ultima,tely to reach, and must indeed be got rid of before that 

 final condition can be attained. 



Cases of this kind forcibly illustrate the necessity for some 

 explanation of the facts of development. Much attention has 

 been given to the subject, especially of recent years, and it is 

 now possible to frame a consistent theory which will explain the 

 general history of development in all groups of animals, and 

 which will also be in harmony with the accepted views con- 

 cerning the mutual relations of these groups. 



The doctrine of descent, or of evolution, teaches us that as 



