10 CLASSIFICATION AND ADAPTATION 



may be produced by the same parents, or which are 

 merely stages in the life of the individual. There 

 are cases in which the limits of species or the 

 boundaries between them are indistinct, where there 

 is a graduated series of differences through a wide 

 range of structure, but these cases are the exception ; 

 usually there are a vast majority of individuals 

 which belong distinctly to one species or another, 

 while intermediate forms are rare or absent. The 

 problem then is, How did these distinct species arise ? 

 How are we to explain their relations to one another 

 in groups of species or genera ; why are the genera 

 grouped into families, families into orders, orders 

 into classes, and so on ? 



There are thus two main problems of evolution: 

 first, how have animals become adapted to their 

 conditions of life, how have their organs become 

 adapted to the functions and actions they have to 

 perform, or, at least, which they do perform ? The 

 power of flight, for example, has been evolved by 

 somewhat different modifications in several different 

 types of animals not closely related to one another : 

 in reptiles, in birds, and in mammals. We have no 

 reason to believe that this faculty was ever uni- 

 versal, or that it existed in the original ancestors. 

 How then was it evolved ? The second great prob- 

 lem is, How is it that existing animals, and, as the 

 evidence of the remains of extinct animals shows, 

 these that existed at former periods of time also, 

 are divided into the groups or types we call species, 

 naturally classified into larger groups which are 

 subdivisions of others still larger, and so on, in what 

 we call the natural system of classification ? The 

 two problems which naturalists have to solve, and 



