258 



HABIT AND INTELLIGENCE. 



[chap. 



Kidneys 

 same. 



Brain of 



human 



embryo. 



Develop' 

 ment of 

 spinal 

 column. 



of embryos of mammalia and of birds, whicb are homo- 

 logous with the kidneys of fishes. Unlike the branchial 

 arteries, tliese perform their function at first : they act as 

 kidneys until, before birth, they are superseded by the 

 permanent kidneys.-^ 



In the organs belonging to the nervous system, there is 

 no case whatever, I believe, of any change in the plan of 

 development : but, notwithstanding, the first embryonic 

 brain of the highest animals is not a miniature likeness of 

 its mature state ; on the contrary, the human brain, when 

 it can first be distinguished, is very like that of a fish ; 

 but, in conformity with the law already mentioned, it is 

 more like that of the embryonic fish than that of the 

 mature one. And, as the successive stages of development 

 in the higher Batrachians correspond to the ascending- 

 series of specific forms among the lower Batrachians, and 

 as the successive stages of development in the branchiae of 

 the higher Crustacea correspond to the ascending series of 

 forms among the lower Crustacea, so do the stages of the 

 development of the human brain correspond to the brains 

 of the ascending series of mammalian forms.^ This kind 

 of correspondence, indeed, is a general law. One of the 

 best instances of it is the development of the spinal 

 column of Vertebrates, which begins in all as a mere tube 

 of gelatinous substance, and afterwards becomes seg- 

 mented : presenting successively, in the course of its 

 development among the higher Vertebrata, the likeness 

 of different forms which are those of mature forms among 

 the inferior orders.^ 



Another instance may be mentioned, of what we may call 

 physiological development, as distinguished from morpho- 

 logical. Bed blood-corpuscles are found in the Vertebrata 

 only, and are all but universal in their mature forms : ^ but 



^ Carpenter's Human Physiology, p. 809 . 



2 For these facts about the development of the brain, see ibid. p. 823. 



3 Carpenter's Comparative Physiology, p. 178. 

 * The only known exception is the amphioxus, which is the lowest of all 



fishes. It ought to be mentioned that some worms have red blood, but 

 not red blood-corpuscles : the red colour is due to some difiiised colotiring 

 matter. 



