200 THE STUDY OF INSECTS. 



history of the various races of animals and plants, or phylogeny (phy- 

 log'e-ny), as it is termed. 



The scope of this book has not permitted an extended treatment 

 of this phase of the subject. There is space for only a few hints re- 

 garding the phylogeny of the families of a single order; but these 

 hints will serve as an illustration of a method of study. The Lepi- 

 doptera is chosen for this purpose, as the method has been applied to 

 this order more fully than it has to others. 



It is a well-known fact that every kind of animal and plant trans- 

 mits a general likeness with individual differences to its offspring. 



According to the Darwinian theory of natural selection these dif- 

 ferences or variations may be of any kind and in any direction. And 

 as many more animals are born or plants germinated than can live to 

 reach maturity, owing to the tendency of each kind to increase in a 

 geometrical ratio, each individual is subjected to a severe struggle for 

 existence. 



The result of this struggle is that any individual possessing a for- 

 tunate variation — that is, one that enables it to get its living and 

 escape its dangers more easily than its fellows — will be more apt on 

 this account to reach maturity and propagate its kind than will less 

 fortunate individuals. Thus there is a thinning-out process which 

 tends to the production of more and more specialized forms of animals 

 and plants, i.e., forms adapted to the special conditions under which 

 they exist. 



It should be remembered that the difficulties surrounding exist- 

 ence may be met in different ways ; and that thus there may have 

 descended from a common ancestor very different forms, each well 

 fitted to meet the struggle for existence. See Chapter I, pp. i and 2. 



Just what changes have taken place in the structure of the mem- 

 bers of any race is a difficult matter to determine, for, although many 

 fossils have been found, the record is still very incomplete. But for- 

 tunately something can be learned regarding this by the study of 

 living animals. For not all members of the same family, or order, or 

 class are equally specialized. Some retain more nearly than others 

 the form of their remote ancestors ; and by the study of these general- 

 ized ioxm.<s>, as they are termed, we can gain some idea of the struc- 

 ture of the animals of past ages, and of the ways in which existing 

 animals have been modified. 



We will state very briefly some of the conclusions that we have 

 reached regarding the phylogeny of the families of the Lepidoptera. 

 These conclusions are based largely on a study of the wings. It is 

 hoped that other parts will be studied in the same way ere long. 



