NASCENT ORGANS. 83 



animals, seem there to foreshadow the coming of 

 homologous organs, highly developed in higher 

 animals. As good an example as can be chosen of a 

 nascent organ, is the air-bladder of fishes. It is of 

 very little use to the fish, or, at all events, is only 

 valuable for a hydrostatic apparatus ; but it occurs in 

 such relations as to show that it is the undoubted 

 homologue of the lungs of land animals. Occurring 

 in low animals, while lungs only belong to those 

 more highly developed, it cannot be regarded as 

 the aborted remnant of a previously functional 

 lung, for the ancestors of fishes never possessed 

 lungs. Agassiz considered them to be prophecies of 

 the coming lungs; an idea in perfect accordance 

 with his general theory of types existing in the 

 Creator's mind, but of no meaning otherwise. 



But the theory of descent has also its explanation 

 of these cases, which is as intelligible, and has the 

 advantage of being natural rather than supernatural. 

 These organs are considered not as remnants but as 

 beginnings. They are new organs, capable of fur- 

 ther development, and not organs on their way 

 toward disappearance. The theory claims, more- 

 over, that they are not functionless, like those re- 

 sulting from disuse, but are of some value to their 

 possessor, although the value may be a slight one. 

 Every organ, according to the descent theory, must 

 make its appearance as a very simple structure, 

 which gradually develops, generation after genera- 

 tion, and in the course of time may become very 

 greatly developed. A very simple organ in a low 

 animal may therefore evidently be some such rudi- 



