154 THE DOCTRINE OF DESCENT. 



ment 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 demon- 

 strated, even in their minutest microscopic details. If 

 the idea of design as a principle ' of explanation is ex- 

 cluded 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 

 processes 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 circum- 

 stances." Perhaps this maxim may in future be brought 

 into play, for it is actually a question of investigating 

 how surrounding conditions, the agencies acting on the 

 sensory apparatus, can exercise on simple matter such 

 an influence, that the otherwise widely differing descend- 

 ants 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 con- 

 struction. Darwinism has never yet pretended to have 

 explained everything; neither will it be wrecked on this 

 point, but, on the contrary, 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 Vertebrata; 

 in this instance, however, it does not go beyond an anal- 



