154 THE DOCTRINE OF DESCENT. 



when the development of their optic apparatus had not 

 attained the degree of perfection which we now find in 

 the stallv-eyed crabs and insects. Tliey nevertheless 

 agree not merely in their coarser conditions, but, as 

 Max Schultze has demonstrated, even in their minutest 

 microscopic details. If the idea of design as a principle 

 of explanation is excluded in this case also, as will be 

 shown below, and as is self-evident from our standpoint, 

 and if simple heredity in the two series must be excluded 

 likewise, some other adequate solution must be sought. 



The case of the converging species of sponges may 

 throw a light, feeble though it be, upon the obscure pro- 

 cesses of the organic laboratory. Let us here again 

 recall that maxim of Goethe, which we have already 

 cited : " The animal is formed by circumstances for 

 circumstances." Perhaps this maxim may in future be 

 brought into play, for it is actually a question of in- 

 vestigating how surrounding conditions, the agencies 

 acting on the sensory apparatus, can exercise on simple 

 matter such an influence, that the otherwise widely 

 differing descendants of the various possessors of this 

 simple material or incomplete organs, have acquired a 

 more complete organ, not only working in a similar 

 manner, but of similar construction. Darwinism has 

 never yet pretended to have explained everything ; 

 neither will it be wrecked on this point, but, on the con- 

 trary, will only have supplied fresh incitements to more 

 profound researches, crowned by beautiful results. 



Another example of approximation in divergent 

 scries is afforded by the eyes of the highest molluscs, 

 the Cephalopods, as compared with those of the Ver- 

 tebrata ; in this instance, however, it does not go 



