6 RETROGRESSIVE DEVELOPMENT IN NATURE, [IX. 



It is not only among the ostriches that we find degeneration 

 of this kind ; certain species of water-birds have become too 

 heavy and awkward to rise into the air, and in these too, for 

 instance in the penguin, the wings are quite useless as organs of 

 flight. But, although useless for flying, they are still of some 

 service for motion in water, and therefore have not degenerated 

 as completely as those of the Kiwi. They have, however, 

 become far smaller than those of flying birds, and, clothed with 

 short scale-like feathers, they bear some resemblance to the 

 fins of fishes. 



These few instances will suffice to show that nature is pur- 

 poseful, not only in adapting recently developed structures to 

 her uses, i. e. in fitting them to perform properly the functions 

 allotted to them, but, conversely, in removing everything 

 superfluous, so that as soon as a structure is no longer required 

 it is eliminated. Of course, this elimination is neither sudden '■ 

 nor voluntary, but comes to pass gradually, in accordance with 

 certain laws, so that we are often able to watch every stage 

 of the transition from the full development of an organ to the 

 entire absence of it. 



Such degeneration of once important parts is not only found 

 here and there in nature ; it is of frequent, nay, among the 

 higher animals, of general occurrence. It is in fact a natural 

 consequence of the evolution of the higher animals of to-day 

 from earlier and lower forms, which lived under totally different 

 conditions, necessitating the possession of parts and organs, 

 which, in process of time, have been either altered or com- 

 pletely atrophied. If nature had not possessed the power to 

 cause the disappearance of superfluous organs, there could 

 have been no such thing as the transmutation of species ; for 

 primitive structures, when they became superfluous, would have 

 been in the way of those in actual use and would have hindered 

 their development. Indeed, had the retention of all original 

 structures been a necessity from the first, the result would 

 have been the production of monsters quite unfit to live. 

 Hence the retrogression of superfluous structures is a condition 

 of progression. 



Having found disuse to be the immediate cause of the dis- 

 appearance of a structure in the course of the development of 

 a species, we may further ask how a structure once essential to 



