186 APPENDIX II—INTENSIVE SEGREGATION. 
1. Separation always Involves more or less Segregation, for no two Portions of a 
Species Possess exactly the Same Average Character. 
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 proportionsin which they are produced by 
the main body of the species, or by any other small colony thatis 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 specumens possessing an average character im any one 
respect are always abundant, those perfectly representing the average in 
every respect are rarely, tf ever, found. 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. 
