154 A BOOK OF INSECTS 



to differ so strikingly in colour and shape from the typical 

 members of its family. At first thought we may fail to 

 find in natural selection an adequate explanation of a case 

 so remarkable. But we must remember that we are look- 

 ing at the latest result of a work which may have been 

 in progress for many thousands of centuries. In the 

 beginning, both the models and the mimic were probably 

 much more simple in colouring than they are to-day. 

 Then, as the models became more elaborate, the mimic 

 followed suit, diverging gradually under the guidance of 

 natural selection from the typical members of its family. 

 Nor is it possible to dogmatise as to the degree of varia- 

 tion upon which natural selection works most fruitfully. 

 We know that every offspring of a given insect differs in 

 minute details from its parents, yet we need not infer that 

 these slight differences afford the only openings for profit- 

 able selection. From time to time varieties arise that are 

 glaringly unlike the ancestral type. Great and sudden 

 jumps, as it were, are taken by Nature. Moreover, it 

 is noticeable that certain species are especially liable to 

 vary in this extreme degree, and that the same kinds of 

 varieties (mutations as they are called) are produced over 

 and over again. In all such cases interbreeding among 

 similar varieties must occur ; and some naturalists (who 

 follow Professor De Vries of Amsterdam) assert that all 

 species pass through these periods of instability in the 

 course of their history — periods when swarms of incipient 

 species arise suddenly from the old stock. On this as- 

 sumption the profound modification of an existing species, 

 or the making of new ones, might be a comparatively 

 rapid process ; for while many of the mutations are des- 

 tined to fall before the ordeal of natural selection, a few 

 may pass muster and transmit their distinctive characters 

 to posterity. 



