The Causes of Individual Differences 15 



the methods or reasons of their origin. It is possible that, 

 whilst many of these mere individual differences have no 

 direct and immediate cause, they may still be the result of 

 a devious line of antecedent causes long since so much 

 diffused and modified that they will remain forever un- 

 recognizable ; but even so, the fact still remains that 

 these present differences or variations may be purposeless, 

 and it is quite as well to say that they exist because it is a 

 part of the organic constitution of living things that un- 

 like produces unlike. 



Sex as a factor in the variation of plants. All plants 

 have the faculty, either potential or expressed, of propagat- 

 ing themselves by means of buds, or asexual parts. This is 

 obviously the cheapest and most direct possible method 

 of propagation for many-membered plants, since it re- 

 quires no special reproductive organization and energy, 

 and, as only one parent is concerned in it, there is none of 

 the risk of failure that obtains in any mode of propaga- 

 tion in which two parents must find each other and form 

 a union. There must be some reason, therefore, for the 

 existence of such a costly mechanism as sex aside from 

 its use as a mere means of propagation. 



It may be said that sex exists because it is a means of 

 more rapid multiplication than bud-propagation, but such 

 is not necessarily the fact. Many plants produce buds as 

 freely as they produce seeds ; and then, if mere multipli- 

 cation were the only destiny of the plant, bud-production 

 would no doubt have greatly increased to have met the 

 demand for new generations. The chief reason for 

 the existence of sex in the vegetable world seems to be the 

 need for a constant rejuvenation and modification of the 



