EVOLUTION 367 



inherited by the offspring." A creature which inherits 

 favourable variations " will have a better chance of surviving 

 and thus be naturally selected.'^ 



The theory of natural selection is not difficult to under- 

 stand, and the process is certainly going on among insects 

 as among all other living creatures. In every chapter of the 

 present volume we have noticed examples of manifold ways 

 in which insects are adapted in their structure and behaviour 

 to their surroundings and to the conditions of their lives. 

 Darwin, in his study of the problem of evolution, regarded 

 a clear insight into the means of modification and co- 

 adaptation to be of the greatest importance, and became 

 " convinced that Natural Selection has been the main but 

 not exclusive means of modification." Some of his disciples 

 in the latter part of last century went far beyond this 

 characteristically moderate pronouncement of their master 

 by advocating the " all-sufficiency of natural selection." 

 A reaction against this extreme view has gathered strength 

 during the last twenty-five years concurrently with an 

 advance of our knowledge of the details of inheritance and 

 variation, so that such expressions as the " bankruptcy " 

 of the Darwinian theory are not unknown in some quarters 

 to-day. 



Insects form a group of creatures the study of which may 

 be especially helpful in discussions about adaptation. The 

 rate of multiplication of many insects is so rapid that they 

 afford striking examples of the " high geometrical ratio of 

 increase " on which Darwin laid stress. Linne, long ago 

 noticing the rapid reproduction of the blow-flies whose 

 maggots feed in flesh, remarked that three of these insects 

 could devour more quickly than a lion the carcase of a 

 horse. Huxley calculated that, if all the progeny of a 

 '* stem-mother " aphid survived, and left offspring which 

 all in their turn survived to propagate their kind, and so on 

 through the successive generations of spring and summer, 

 the descendants of that one stem-mother at the end of the 

 season would collectively exceed in weight the whole human 

 population of China ! 



