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 stalk-eyed crabs and insects. They 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 
series 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 
