146 VESTIGES OF THE 



the moUusca and articulata being, as it were, two distinct 

 parallel lines by which we pass from the radiata to the 

 vertebrata. The gradation can, in like manner, be clearly 

 traced in the classes into which the sub-kingdoms are 

 subdivided, as, for insttmce, when we take those of the 

 vertebrata in this order — fishes, reptiles, birds, mammals. 



While the external forms of all these various animals 

 are so ditierent, it is very remarkable that the whole are, 

 after all, variations of a fundamental plan, which can be 

 traced as a basis throughout the whole, the variations 

 being merely modifications of that plan to suit the par- 

 ticular conditions in which each particular animal has 

 been designed to live. Starting from the primeval germ 

 which, as we have seen, is the representative of a par- 

 ticular order of full-grown animals, we find all others to 

 be merely advances from that type, with the extension of 

 endowments and modification of forms which are required 

 in each particular case; each form, also, retainmg a 

 strong afiinity to that which precedes it, and tending to 

 impress its own features on that which succeeds. This 

 unity of structure, as it is called, becomes the more 

 remarkable when we observe that the organs, while 

 preserving a resemblance, are often put to different uses. 

 For example : the ribs become, in the serpent, organs of 

 locomotion, and the snout is extended, in the elephant, 

 into a prehensile instrument. 



It is equally remarkable that analogous purposes are 

 served in different animals by organs essentially different. 

 Thus, the mammalia breathe by lungs ; the fishes, by 

 gills. These ai-e not modifications of one organ, but 

 distinct organs. In mammifers. the gills exist and act 

 at an eai-ly stage of the fcctal state, but aftei-wards go 

 back and appear no more ; while the lungs are developed. 

 In fishes, agjiin, the gills only are fully developed ; while 



