Useful Variations 27 



But even in regard to this case we are reasoning in a circle, not 

 giving "proofs/' and no one who does not wish to believe in the 

 selection-value of the initial stages can be forced to do so. Among 

 the many pieces of presumptive evidence a particularly weighty one 

 seems to me to be the smallness of the steps of progress which we 

 can observe in certain cases, as for instance in leaf-imitation among 

 butterflies, and in mimicry generally. The resemblance to a leaf, 

 for instance of a particular Kallima, seems to us so close as to be 

 deceptive, and yet we find in another individual, or it may be in 

 many others, a spot added which increases the resemblance, and which 

 could not have become fixed unless the increased deceptiveness so 

 produced had frequently led to the overlooking of its much persecuted 

 possessor. But if we take the selection-value of the initial stages for 

 granted, we are confronted with the further question which I myself 

 formulated many years ago : How does it happen that the necessary 

 beginnings of a use/id variation are always present ? How could 

 insects which live upon or among green leaves become all green, 

 while those that live on bark become brown ? How have the desert 

 animals become yellow and the Arctic animals white? Why were 

 the necessary variations always present ? How could the green locust 

 lay brown eggs, or the privet caterpillar develop white and lilac- 

 coloured lines on its green skin? 



It is of no use answering to this that the question is wrongly 

 formulated 1 and that it is the converse that is true; that the 

 process of selection takes place in accordance with the variations 

 that present themselves. This proposition is undeniably true, but so 

 also is another, which apparently negatives it : the variation required 

 has in the majority of cases actually presented itself. Selection can- 

 not solve this contradiction ; it does not call forth the useful variation, 

 but simply works upon it. The ultimate reason why one and the 

 same insect should occur in green and in brown, as often happens in 

 caterpillars and locusts, lies in the fact that variations towards brown 

 presented themselves, and so also did variations towards green : the 

 kernel of the riddle lies in the varying, and for the present we can 

 only say, that small variations in different directions present them- 

 selves in every species. Otherwise so many different kinds of 

 variations could not have arisen. I have endeavoured to explain 

 this remarkable fact by means of the intimate processes that must 

 take place within the germ-plasm, and I shall return to the problem 

 when dealing with "germinal selection." 



We have, however, to make still greater demands on variation, 

 for it is not enough that the necessary variation should occur in 

 isolated individuals, because in that case there would be small 



1 Plate, Selektionsprinzip u. Probleme der Artbildung (3rd edit.), Leipzig, 1908. 



