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 thes z.general¬ 
ized forms, 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. 
