58 THE EVOLUTION THEORY 



ther-e is, furthermore, the difficulty of estimating the biological 

 importance of any variation that may occur. Even in cases where 

 we know its significance quite well in a general way, we cannot 

 estimate its relative value in reference to the variation of some other 

 character, though that other may also be quite intelligible. Later 

 on, we shall speak of protective colouring, and in so doing we shall 

 discuss the caterpillars of one of the Sphingidse, which occur in two 

 protective colours, some being brown, others green. From the greater 

 frequency of the brown form we may conclude that brown is here 

 a better adaptation than green, but how could we infer this from 

 the character itself, or from our merely approximate knowledge of 

 the mode of life of the species, its habits, and the dangers which 

 threaten it ? A direct estimation of the relative protective value of 

 the two colours is altogether out of the question. The survival of the 

 fittest cannot be proved in nature, simply because we are not in 

 a position to decide, a priori, what the fittest is. For this reason 

 I was forced to try to make the process of natural selection clear by 

 means of imagined examples, rather than observed ones. 



But though we cannot directly follow the uninterrupted process 

 of natural selection which is going on under natural conditions, there 

 is another kind of proof for this hypothesis, besides that which 

 consists in logically deducing a process from correct premisses; I should 

 like to call this the practical proof. If a hypothesis can be made 

 to explain a great number of otherwise unintelligible facts, it thereby 

 gains a high degree of probability, and this is increased when there 

 are no facts to be found which are in contradiction to it. 



Both of these criteria are fulfilled by the selection-hypothesis, 

 and indeed the phenomena which may be explained by it, and are 

 intelligible in no other way, present themselves to us in such enormous 

 numbers, that there can be no doubt whatever as to the correctness of 

 the principle ; all that can be still disputed is, how far it reaches. 



Let us now turn our attention to this practical way of proving 

 the theory by the facts which it serves to interpret, beginning with 

 a consideration of the external appearance of organisms, their colour 

 and form. 



The Colour and Fovm of Organis'ms. 



Erasmus Darwin had in many cases already rightly recognized 

 the biological significance of the colouring of an animal species, and 

 we may be sure that many of the numerous good observers of earlier 

 times had similar ideas. I can even state definitely that Rosel von 

 Rosenhof, the famous miniature-painter and naturalist of Niirnberg 



