INTRODUCTION 15 



conditions accumulated by heredity. The only reason for 

 rejecting this conclusion is the belief that the direct effects 

 of conditions are not inherited. But this is only a belief, not 

 an established truth. 



Now it is possible by actual observation to ascertain what 

 evidence there is that variations which might by natural 

 selection be combined into the adaptations we see, do occur 

 apart from the special habits or conditions to which the 

 adaptations are related. The variations that occur constantly 

 in the form of individual differences have been minutely 

 investigated in the past few years by statistical methods with 

 the aid of the higher mathematics. The greater the differ- 

 ence the more rarely it occurs, and occasionally striking 

 abnormalities are observed, the character of which points to 

 definite principles of symmetry and repetition in develop- 

 ment. But it is not proved that, without change of condi- 

 tions, variations occur which could, by selection, give rise to 

 such special adaptations as abound in the animal kingdom. 

 For example the power of partial or complete flight by means 

 of a membranous fold of skin has been evolved in many 

 independent cases in the vertebrate sub-kingdom, in the 

 extinct pterodactyle reptiles, in bats, in flying squirrels, and 

 in flying marsupials. But the variations in the condition of 

 the skin, in animals that do not fly or take long leaps through 

 the air, are not such as to justify the belief that these varia- 

 tions would make any difference in the struggle for existence 

 when long leaps became necessary. Unless the differences 

 were great enough to cause the comparative success of certain 

 individuals and the comparative failure of others, the process 

 of natural selection could not commence. On the other 

 hand, the very condition which the principle of natural 

 selection in this case implies, the necessity or advantage of 

 long leaps or incipient flight, involves the practice of new and 

 special actions and movements. Such new habits must have 



