100 HYPOTHESIS OF THE DEVELOPMENT OF 



of full-grown animals, we find all others to be merely 

 advances from that tvpe, with the extension of endow- 

 ments and modification of forms which are required ir? 

 each particular case; each form, also, retaining a strong 

 affinity 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 re- 

 semblance, 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 are not modifications of one organ, but distinct or- 

 gans. In mammifers, the gills exist^md act at an early 

 stage of the foetal state, but afterward go back and appear 

 no more ; while the lungs are developed. In fishes, again, 

 the gills only are fully developed ; while the lung struc- 

 ture either makes no advance at all, or only appears in the 

 rudimentary form of an air-bladder. So, also, the baleen 

 of the whale and the teeth of the land mammalia are dif- 

 ferent organs. The whale, in embryo, shows the rudi- 

 ments of teeth ; but these, not being wanted, are not de- 

 veloped, and the baleen is brought forward instead. The 

 land animals, we may also be sure, have the rudiments of 

 baleen in their organization. In many instances, a par- 

 ticular structure is found advanced to a certain point in a 

 particular set of animals (for instance, feet in the serpent 

 tribe), although it is not there required in any degree; 

 but the peculiarity, being carried a little farther forward, 

 is perhaps useful in the next set of animals in the scale. 

 Such are called rudimentary organs. With this class of 

 phenomena are to be ranked the useless mamma? of the 

 male human being, and the unrequired process of bone in 

 the male opossum, which is needed in the female for sup- 

 porting her pouch. Such curious features are most con- 

 spicuous in animals which form links between various 

 classes. 



As formerly stated, the marsupials, standing at the bot- 

 tom of the mammalia, show their affinity to the oviparous 

 vertebrata, by the rudiments of two canals passing from 

 near the anus to the external surfaces of the viscera, which 



