VI 
DIFFICULTIES AND OBJECTIONS 
149 
soil, climate, and atmosphere widely different from those of 
their native habitat. Thus, many alpine plants only found 
near perpetual snow thrive well in our gardens at the level of 
the sea; as do the tritomas from the sultry plains of South 
Africa, the yuccas from the arid hills of Texas and Mexico, and 
the fuchsias from the damp and dreary shores of the Straits of 
Magellan. It has been well said that plants do not live 
where they like, but where they can ; and the same remark will 
apply to the animal world. Horses and cattle run wild and 
thrive both in North and South America; rabbits, once con¬ 
fined to the south of Europe, have established themselves in 
our own country and in Australia; while the domestic fowl, a 
native of tropical India, thrives well in every part of the 
temperate zone. 
If, then, we admit that when one portion of a species is 
separated from the rest, there will necessarily be a slight 
difference in the average characters of the two portions, it 
does not follow that this difference has much if any effect 
upon the characteristics that are developed by a long period 
of isolation. In the first place, the difference itself will 
necessarily be very slight unless there is an exceptional 
amount of variability in the species; and in the next place, 
if the average characters of the species are the expression of 
its exact adaptation to its whole environment, then, given 
a precisely similar environment, and the isolated portion will 
inevitably be brought back to the same average of characters. 
But, as a matter of fact, it is impossible that the environment 
of the isolated portion can be exactly like that of the bulk of 
the species. It cannot be so physically, since no two separated 
areas can be absolutely alike in climate and soil; and even if 
these are the same, the geographical features, size, contour, and 
relation to winds, seas, and rivers, would certainly differ. 
Biologically, the differences are sure to be considerable. The 
isolated portion of a species will almost always be in a much 
smaller area than that occupied by the species as a whole, hence 
it is at once in a different position as regards its oavii kind. 
The proportions of all the other species of animals and plants 
are also sure to differ in the two areas, and some species will 
almost always be absent in the smaller which are present in 
the larger country. These differences will act and react on 
