30 RETROGRESSIVE DEVELOPMENT IN NATURE. [IX. 



the frog is but an imperfect organ of vision as compared with 

 the eye of the falcon, or that of man, but it is perfect enough to 

 enable it to see the crawling fly or the writhing worm : it 

 suffices for the needs of the species. Even the eye of the 

 falcon is not absolutely perfect as an organ of vision from a 

 purely optical point of view, but it serves to enable the bird to 

 distinguish its prey with certainty from a great height : such a 

 pitch of perfection is all that is essential for the life of the 

 species, and all possibility of higher development of the eye, by 

 means of natural selection, is therefore precluded. The object 

 of all evolution, viz. the survival of the fittest, is not, however, 

 always and only attained by the ever-improving, progressive 

 development of the organism as a whole, or of particular 

 organs : new possessions are not invariably added to the old, 

 but the latter are often rendered superfluous in the course of 

 time and taken away. Nor does this happen in an ideally 

 perfect way, suddenly, as if by magic, but slowly, in accordance 

 with existing laws, so that the process remains uncompleted 

 through long ages. But ultimately the organ which is no 

 longer essential to life is done away with altogether, and the 

 balance between the structure of the body and its functions 

 is restored, so that, in this sense also, retrogression may in 

 truth be said to be a part of progress. 



