324 THE EVOLUTION THEOKY 



This is the case with all adaptations. Just as the eyes of animals 

 are adaptations which utilize the light- waves in the interest of the 

 organism and its survival, the same is true of all the sense-organs, 

 tactile organs, smelling and tracking organs, organs of hearing, and 

 so on. The animal cannot do without these ; first the lower sense- 

 organs arose and then the higher ; the increasingly high organization 

 of the animal conditioned this, and a multicellular animal without 

 sensory structures is inconceivable. The same may be said of the 

 nervous system as a whole, whose function it is to translate into action 

 the stimuli received through the sense-organs, whether directly or by 

 means of intervening nerve-cells, which form central organs of ever- 

 increasing complexity of composition. As telescope-eyes have evolved 

 in some groups of deep-sea animals, independently of one another, 

 and certainly not through the fortuitous occurrence of a mutation, 

 but under the compulsion of necessity in competition, so all the organs 

 we have just named, the whole nervous system with all its sense- 

 organs, must have arisen through the same factors of evolution in 

 numerous independent genealogical lines. And it must not be 

 supposed that this is all ; what is true of the sense-organs — that they 

 are necessities — is undoubtedly true also of all parts and organs of 

 the animal body, both as a whole and in every detail. It cannot be 

 demonstrated in all cases, but it is nevertheless certain that this 

 applies also to all the organs of movement, digestion, and reproduction, 

 to all animal groups and also to the differences between them, even 

 although these may not always be obvious adaptations to the con- 

 ditions of life. What part is left for mutation to play if almost 

 everything is an adaptation ? Possibly the specific difierences ; and 

 these in point of fact cannot in many cases be interpreted with 

 certainty as adaptations, though this can hardly be taken tis a proof 

 that they are not. Possibly also the geometrical skeletons of many 

 unicellulars, in which again we cannot recognize any definite relation 

 to the mode of life. It is easy enough to conceive of the wondrously 

 regular and often very complex siliceous skeleton of the Radiolarians 

 or Diatoms as due to saltatory mutations, and ' leaps ' of considerable 

 ^ magnitude must certainly have been necessary to produce some of the 

 manifold transformations here as everywhere else. But whether 

 these are or are not without importance for the life of the organisms, 

 we are in the meantime quite unable to decide. Here too it is well to be 

 cautious in concluding that these organic ' crystallizations ' are without 

 importance, and therefore to infer that they have arisen suddenly 

 from purely internal causes. One of the experts on Diatoms, F. Schiitt, 

 has shown us that differences in length in the skeletal process of 



