to Lower Animals. 



5 



which are in him never turned to account, but which in the 

 skates, sharks, and dogfish develop gills, and continue to remain 

 open. The same gill-slits exist in the amphibia, namely, in the 

 frogs, newts, and salamanders, during their early stage of 

 development, but ultimately get closed, as in man and all inter- 

 mediate animals. The fish-like heart of the human organism, 

 first single, then double, next assumes the reptilian character of 

 being three-chambered. The skeleton gradually advances from 

 the cartilaginous to the osseous, with elements at first character- 

 istic of all the vertebrata, then becoming more and more special- 

 ised in structure. "At the fourth week," says Topinard, " the 

 difference between the man and the dog is inappreciable. The 

 divergence only commences in earnest at the eighth week." 



The skulls of the higher vertebrates (man included) in the 

 course of their development, are found to pass through stages 

 substantially similar to those at which lower vertebrates are 

 arrested. And the bones composing the skull in higher verte- 

 brates are found to correspond generally with those in lower 

 vertebrates by the position they occupy with reference to the 

 nerves sent out from the brain to the organs of special sense — 

 smell, sight, hearing, &c. 



The elements of the vertebral column and limbs similarly 

 correspond generally in man and the higher vertebrates with the 

 lower vertebrates, as far down even as the amphibia. 



The chief modifications of the brain arise from the develop- 

 ment of the hemispheres in relation to the other parts. The 

 cerebral hemispheres, which are developed from the fore-brain, 

 remain small in the lower vertebrates, while in the higher they 

 generally increase with the ascending status of the animal, 

 extending backward and downward, till at last, in man, they 

 conceal the base of the brain. Moreover, the growth of the 

 hemispheres so outstrips the growth of the skull that the surface 

 of the hemispheres becomes dimpled, then furrowed or wrinkled, 

 which wrinkles become more and more deep and complex as 

 they approach near to man, in whom it is the most distinguishing 

 characteristic, alike for its enormous size aud weight, and the 

 intelligence of which it is the organ. 



So thoroughly similar in character are the elements of which 

 the brain is made up in the higher vertebrates to that found in 

 the lower, that, notwithstanding the many gaps in other respects 

 dividing animals from each other, a gradation of develop- 

 ment is established, which of itself would give presumptive 

 proof of the derivation or evolution of the higher vertebrates 

 from the lower. In vain has it been attempted to discover in 

 the structure of the human brain something which was not found 

 in those of the animals nearest to him. The elements are all 



