RECAPITULATION. 485 



vertebrates, and (2) the absence of any trace, in embryonic 

 life, of the fore-limbs in snakes. 



Embryologists will be aware that the theory so ably 

 advocated by Mr. Sedgwick had already been suggested by 

 Balfour. It is contained in the following paragraph (JZonip. 

 Evibryology, vol. ii., p. 174): — 



" Another point which deserves notice in the snake is 

 the absence in the embryo of all external trace of limbs. It 

 might have been anticipated on the analogy of the branchial 

 arches, that rudiments of the limbs would be preserved in 

 the embryo even when limbs were absent in the adult. 

 Such however is not the case. It is however very possible 

 that rudiments of the branchial arches and clefts have been 

 preserved because these structures were functional in the 

 larva (Amphibia) after they ceased to have any importance 

 in the adult ; and that the limbs have disappeared even in 

 the embryo, because in the course of their gradual atrophy 

 there was no advantage to the organism in their being 

 specially preserved at any period of life." 



Mr. Sedgwick has not extended his criticism to the 

 question of recapitulation in larval stages themselves. 



This however has been done by Mr. E. W. Macbride, 

 who points out that according to the view we have been con- 

 sidering, however modified the record of ancestral history 

 contained in the larval development may be, the embryonic 

 record can never rise above it in value. This writer 

 evidently, whether consciously or unconsciously, takes a 

 Lamarckian view of evolution. He considers that so far as 

 we can judge by comparative anatomy the stimuli to evolu- 

 tion, in the sense of change of structure, have been two ( i ) 

 change of environment or habits, (2) increased or decreased 

 demands on the working of certain organs. Larval stages 

 according to Macbride are due to new stimuli commencing 

 to act at a late period of life ; in the case of flat fishes if the 

 young when they emerge from the ^^^ were at once to 

 adopt the adult mode of life then their symmetrical larval 

 stage would be omitted from their ontogeny. He considers 

 that secondary larva; must be regarded as having arisen 

 owing to the young adults having taken to a new mode ot 



