46 THE THEORY OF NATURAL SELECTION 



which they have to pass their lives is clearly shown 

 by the study of adaptations. This is a subject which 

 those followers of Darwin who believe in the all- 

 sufficiency of natural selection have brought into con- 

 siderable prominence. For a full account of many 

 supposed beautiful adaptations, from the point of view 

 of the most prominent member of the school in ques- 

 tion, reference may be made to Weismann's recently 

 published book, ' The Evolution Theory.' 



On the theory of natural selection in its extreme 

 form, all the parts of an animal or plant or, at any rate, 

 all the points in which one species differs from another 

 nearly related species are supposed to have arisen on 

 account of their usefulness to the creatures possessing 

 them. Every detail of structure is thus regarded as 

 being more or less closely adapted to the circum- 

 stances which attend the life of the animal or plant in 

 question. This adaptation is never, indeed, regarded 

 as perfect, because natural selection is always in 

 progress, and its work is never absolutely done ; 

 but the point is that the features of every part are 

 aimed at some useful purpose ; or, if they are not, 

 then they have been useful in former times and under 

 different circumstances, and are now undergoing a 

 process of gradual removal, because the individuals in 

 which the useless structure is least developed will now 

 have the best chance of surviving. That the form and 

 structure of an animal or plant is in general closely 

 fitted to its environment is of course true ; otherwise 

 the creature would very soon cease to cumber the earth. 

 But the student of adaptation goes into details, and 



