1 6 RETROGRESSIVE DEVELOPMENT IN NATURE. [IX. 



is far from new to many of my readers : it has been so often 

 explained in various well-known works and periodicals, that 

 any further elucidation is unnecessar3\ 



What holds good for the individual as a whole also holds 

 good for each separate organ, inasmuch as the ability of an 

 animal to perform its allotted functions depends on the 

 efficiency of each particular organ : hence, by means of this 

 perpetual elimination of the unfit, every organ is brought to the 

 greatest perfection. On this hypothesis, and on this only, is it 

 possible to explain the wonderful adaptability of the minutest 

 details of structure in animals and plants, and the development 

 of the organic world through the operation of natural forces. 



If this view be the true one, if adaptation in all the parts of 

 living forms be truly the result of natural selection, then the 

 same process which produced these adaptations will tend to 

 preserve them, and they will disappear directly natural selection 

 ceases to act. These considerations show why organs which 

 have become superfluous and have fallen into disuse necessarily 

 degenerate and ultimately disappear. 



As an example of this, let us take one of the newts, which 

 are so common in our swamps and pools in spring. If we 

 examine its eyes we find that they are not ver}^ large, but very 

 highly developed : their structure bears considerable likeness 

 to that of the human eye, and they play a very important part 

 in the fife of the animal, which is almost entirel}'- dependent on 

 keenness of vision for finding its prey. It detects at once and 

 snaps at anything in motion : were it not for its eyes, it would 

 infallibly starve. Now these eyes are extremely delicate and 

 complex organs, which have only very gradually, — i. e. in the 

 course of countless generations and of almost endless time, — 

 reached the degree of perfection attained by them in the living 

 newt. The whole series of developmental stages is not indeed 

 known to us ; but in other groups of animals we find eyes at 

 every grade of development, and from these we can form some 

 idea of the way in which the gradual improvement of an 

 original simple and imperfect eye took place. The slow but 

 steady progress in development from stage to stage is due, 

 as I believe, to the fact that the eyes of these animals were 

 never all exactly alike, nor all equally keen, and that only those 

 individuals survived in each generation in which the develop- 



