14 SEXUAL DIMORPHISM 



conception of its progressive development by the process of 

 selection. It is a matter of observation that any character 

 may be somewhat more or somewhat less developed in 

 individual offspring than in the parents. If the major 

 variations only survive, there will be in the course of genera- 

 tions a definite increase of the character. Different characters 

 leading to survival in different individuals may be combined 

 in the offspring as the result of sexual union. 



But there are two difficulties which generally become 

 evident in applying this explanation to particular cases. 

 Firstly, the difficulty of believing that ordinary differences 

 between individuals of the same species are sufficient to form 

 the starting-point on which selection could act, in producing 

 an adaptation of an extraordinary kind; secondly, the 

 impossibility of proving that the changes produced in the 

 individual organism directly by a change of habits or condi- 

 tions of life had nothing to do with the process of evolution. 

 We know that functional exercise, and external stimulation 

 or irritation do produce great modifications of structure, by 

 causing local growth or local absorption. Constant exercise 

 of special muscles causes enlargement not only of the muscles 

 but of the bones to which they are attached. Broken bones 

 which are not set are known to form new * joints " for them- 

 selves at the point of fracture, and these abnormal articula- 

 tions resemble to a remarkable degree the natural articula- 

 tions. Friction of the epidermis causes it to become horny, 

 and the absence of friction causes it to become thin and soft. 



A special adaptation necessarily implies special habits and 

 special stimulations. These must produce, at least in the 

 individual, corresponding modifications of structure. The 

 peculiarities of special adaptations are generally of the same 

 kind as the modifications which would result from the direct 

 action of the special conditions. The natural conclusion is 

 that the peculiarities in question are the effects of the special 



