38o 
It may be well to notice here a theory by which it is 
supposed that the physical isolation of unchanged variants 
may afford an opportunity for the action of the Natural 
Selection of variations strictly due to sexual reproduction. 
We may suppose, for the sake of argument, that the con 
ditions were similar to those which obtained before isola 
tion took place. In that case, it is assumed that a new 
variety will arise, because the offspring will represent the 
average endowments of the individuals thus isolated. But 
if that were so, varieties would be always arising, inas 
much as all animals are practically isolated within a 
narrow range, apart from physical barriers. In this case, 
the inhabitants of a special locality doubtless intermarry 
within a very narrow limit among an infinitesimally small 
fraction of the whole. And yet this arrangement does not 
affect the general stability of a species, apart from other 
influences. 
The same remarks apply to another cause of isolation. 
There is an old proverb to the effect that “ birds of a 
feather flock together,” and the truth of this familiar 
aphorism is to be found in the fact that variants which 
resemble one another generally segregate themselves from 
the rest of the species. 
“ The editor of the Journal of Horticulture, cSr-v., says, that he can 
keep bantams with the larger breeds without much danger of their 
crossing, but not with the smaller breeds, such as games, Hamburgs, 
&c It has been observed in a district stocked with heavy 
Lincolnshire and light Norfolk sheep, that both kinds, though bred 
together, when turned out, * in a short time separate to a sheep ’; the 
Lincolnshires drawing off to the rich soil, and the Norfolks to their 
own dry light soil; and, as long as there is plenty of grass, ‘ the two 
breeds keep themselves as distinct as rooks and pigeons.’ .... 
With respect to fallow-deer, which live in a semi-domesticated condi 
tion, Mr. Bennett states that the dark and pale coloured herds, which 
have long been kept together in the Forest of Dean, in High Meadow 
Woods, and in the New Forest, have never been known to mingle.”— 
(The Variation, vol. ii., pp. ioi-j.)