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 useful 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! 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 
” Plate, Selektionsprinzip u. Probleme der Artbildung (3rd edit.), Leipzig, 1908. 
