282 COMPARATIVE ANATOMY - 



parachordals and trabeculae cranii, which is afterwards 

 strengthened and replaced to a greater or less degree by 

 membrane and cartilage bones. We should find gill-slits 

 separated by gill-arches in every embryo, and though the 

 gill-slits no longer function as respiratory organs, we should 

 find that the blood-vessels were first correlated to the gill-slits, 

 and therefore had a piscine arrangement, which was gradually 

 altered and modified into the adult condition. Similarly, the 

 history of the brain and spinal cord, and of the excretory and 

 generative organs, would show a close correspondence to the 

 history of the same organs in the frog, but in every case the 

 greater complexity and higher grade of organisation would 

 manifest itself at an earlier or later developmental stage, and 

 would involve some departure from the more simple course of 

 events observed in the frog. This close correspondence 

 between the developmental histories or ontogenies of the 

 members of different classes of vertebrates affords one of 

 the strongest arguments in support of the doctrine of evolution. 

 The fact that a frog, a lizard, a bird, and a mammal, even 

 man himself, passes through a stage in which there are gill- 

 slits, with heart and arteries bearing the same relation to them 

 that they do in a fish, is not, perhaps, proof positive, but is 

 very strong evidence of the descent of each from a fish-like 

 ancestor. In the frog, the fish-like ancestor seems to be 

 revealed to us in the shape of the tadpole. The more we 

 reflect upon it, the more we are convinced that, broadly 

 speaking, ontogeny is a recapitulation of phylogeny, the 

 history of the individual is a synopsis of the history of the 

 race. But though it is true in the main, this principle must 

 not be pushed too far. It would be an error to suppose, for 

 instance, that the tadpole represents, with any degree of 

 exactitude, the fish-like ancestor of the Amphibia. It only 

 represents its general plan and degree of organisation. It 

 would be idle to suppose that changes which have occupied 

 ages could be faithfully represented in a development occupy- 

 ing a few weeks of time, or to ignore the fact that embryos, 

 and even larval forms like the tadpole, exist under very 

 different conditions of life to those under which an ancestor, 

 of a corresponding degree of organisation, lived, and 

 must be modified in many ways to enable them to exist 

 under those conditions. Every individual ontogeny is ab- 



