326 THE BIOLOGY OF INSECTS 



their form from early ancestral conditions. Like all living 

 creatures whose origin as individuals can be studied, they 

 arise from germ-cells borne in the bodies of parents like 

 themselves. If therefore we reject the evolutionary view 

 of living nature that all insect races have, like individuals, an 

 ancestry, we must needs abandon the attempt to explain 

 the method of their origin. And when we find that the 

 insects of past ages, known to us from remains preserved 

 as fossils in rocks of various periods, differ on the whole 

 from insects of to-day the more markedly the farther the 

 history of the Class is traced back in geological time, we 

 are confirmed in the belief that in " descent with modifica- 

 tion " — an age-long series of countless generations through 

 which changes in form, in methods of growth, in ways of 

 behaviour, have been worked out — we have a clue to the 

 problems presented by the life of insects as by the whole 

 realm of organised creatures. 



It is easy, then, to accept the general principle of evolution 

 as w^e seek to understand in a wide view the Biology of 

 Insects, but to work out the principle in detail to fit the 

 immense extent of varied facts and relations becomes in- 

 creasingly difficult. Insects are of high im.portance in 

 discussion on these subjects because so many factors, real 

 or supposed, in the general evolutionary progress of living 

 beings have been elucidated through the study of insect life. 

 Considerations of space preclude the survey in this chapter 

 of a great mass of detail, however interesting such may be. 

 We must, however, attempt to discuss in outline, with 

 detail sufficient for elucidation, the two great evolutionary 

 problems about which much difference of opinion has pre- 

 vailed and still prevails among students. These problems 

 may be defined as (i) The Course of Evolution ; and 

 (2) The Factors or Methods of Evolution, among Insects. 

 These problems are not absolutely separable, since various 

 facts and inferences which throw light on the one may be 

 of service in elucidating the other ; it will be convenient, 

 however, to consider them in turn, indicating in some degree 

 how they are correlated. 



