ORIGIN OF THE SPECIFIC TYPE 323 



areas an abundance of food for these mud -eaters, it is only partly- 

 illuminated, that is, only in places where there are luminous animals 

 such as polyp -colonies, &c. The fact that so many of the animals 

 of the great depths are luminous obviously conditions, not that most 

 of the immigrants into the abyssal zone should lose their eyes as 

 useless, but that they should adapt them to the light which is very 

 weak in comparison with that of the superficial layers. The eyes of 

 deep-sea fishes, for instance, are either enormously large, and therefore 

 suited for perceiving the faint light of the depths, or they have varied 

 in another and very characteristic manner: they have become 

 elongated into a cylinder, which projects far beyond the level of the 

 head. It looks almost as if the animals were looking through an 

 opera-glass, and Chun has called these eyes ' telescope-eyes.' A. Brauer 

 has recently shown what far-reaching variations of the original eye 

 of fishes were necessary in order to transform it into an organ for 

 seeing in the dark. These variations, however, have occurred in the 

 eyes of the most diverse animals in the deep sea, and not only do 

 different families of deep-sea fishes possess ' telescope-eyes,' but Crus- 

 taceans and Cephalopoda as well. Even our owls possess quite a 

 similar structure, although it does not project beyond the head in the 

 same way. Here again we have to deal with the phenomenon which 

 Oscar Schmidt in his time called convergence, that is, corresponding 

 adaptations to similar conditions in animal forms not genealogically 

 connected with one another. These telescope-eyes are not all descended 

 from one species which chanced in one of the ' mutation -periods ' sud- 

 denly to produce this combination of harmonious adaptations, but 

 they have risen independently through variation progressing step by 

 step in the direction of the required end, that is to say, through 

 natural selection based upon germinal selection. Only thus can their 

 origin be understood. 



But what is true of eyes adapted to darkness is true in some 

 measure of all eyes, for the eyes of animals are not mere decorative 

 points which might be present or absent; they cannot have arisen 

 in any animal whatever through sudden mutation — they have been 

 laboriously acquired with difficulty, by the slow increase of gradually 

 perfecting adaptations ; they are parts which bear the most precise 

 internal correlation with the whole organization of the animal, and 

 which can only cease to exist when they become superfluous. Thus 

 the origin of eyes seems to me only conceivable on the basis of 

 germinal selection controlled towards what is purposeful by natural 

 selection, that is to say, on a basis of fluctuating variation, and not 

 through chance. 



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