1 86 APPENDIX II INTENSIVE SEGREGATION. 



1 . Separation always Involves more or less Segregation, for no two Portions o{ a 

 Species Possess exactly the Same Aierage Ctiaracter. 



When a homogenous species is divided into two large sections, it 

 may be difficult to prove by measurement that there is any difference 

 in their average character ; but on general principles we may assume 

 that, at least in some points, there is a slight difference. It is evident 

 that when the separated sections are small there is more likely to be 

 diversity in the average character of the sections and that, roughly 

 stated, the probability of divergence from this cause will be in direct 

 proportion to the variableness of the species and in inverse proportion 

 to the size of the different sections. When a few stragglers form a 

 small colony in an isolated position there is the strongest reason to 

 expect that they will not be able to propagate the characters of the 

 species in exactly the same proportions in which they are produced by 

 the main body of the species, or by any other small colony that is prop- 

 agating independently ; and when the original stock has been rendered 

 highly variable by the crossing of somewhat divergent varieties, the 

 degree of difference that will probably be presented by any two inde- 

 pendent colonies will be correspondingly increased. We must bear in 

 mind that while specimens possessing an average character in any one 

 respect are always abundant, those perfectly representing the average in 

 every respect are rarely, if ever, jound. Now, is it to be supposed that 

 any one or any small number of these imperfect representatives of a 

 species will, if separated from the rest, transmit all the characteristics 

 of that species in the exact proportions presented by the average char- 

 acter of the original stock? 



Mr. Francis Galton has conclusively shown* that in the children of 

 parents whose heights deviate from the average of the race to which 

 they belong, there will be a similar deviation amounting on the average 

 to a certain fixed proportion of that presented by what he calls the 

 mid-parentage. The mid-filial deviation in the groups investigated 

 by him was about two-thirds of the mid-parental deviation. There 

 is, therefore, a regression in the average character of the offspring 

 toward the typical character of the group. It must be observed, how- 

 ever, that this law can hold in full force only when there has been free 

 crossing, for otherwise there will be no type from which the deviation 

 can be measured. 



* See "Types and Their Inheritance," an address before the Section of Anthro- 

 pology of the British Association in 1885; also "Natural Inheritance," p. 97. 



