166 
DARWINISM 
CHAP. 
Parallelism between Crossing and Change of Conditions. 
In the whole series of these phenomena, from the beneficial 
effects of the crossing of different stocks and the evil effects of 
close interbreeding, up to the partial or complete sterility 
induced by crosses between species belonging to different 
genera, we have, as Mr. Darwin points out, a curious parallelism 
with the effects produced by change of physical conditions. 
It is well known that slight changes in the conditions of life 
are beneficial to all living things. Plants, if constantly grown 
in one soil and locality from their own seeds, are greatly 
benefited by the importation of seed from some other locality. 
The same thing happens with animals ■ and the benefit we our¬ 
selves experience from “ change of air ” is an illustration of 
the same phenomenon. But the amount of the change which 
is beneficial has its limits, and then a greater amount is 
injurious. A change to a climate a few degrees warmer or 
colder may be good, Avhile a change to the tropics or to the 
arctic regions might be injurious. 
Thus we see that, both slight changes of conditions and 
a slight amount of crossing, are beneficial; while extreme 
changes, and crosses between individuals too far removed in 
structure or constitution, are injurious. And there is not 
only a parallelism but an actual connection between the two 
classes of facts, for, as we have already shown, many species 
of animals and plants are rendered infertile, or altogether 
sterile, by the change from their natural conditions which 
occurs in confinement or in cultivation ; while, on the other 
hand, the increased vigour or fertility which is invariably pro¬ 
duced by a judicious cross may be also effected by a judicious 
change of climate and surroundings. We shall see in a subse¬ 
quent chapter, that this interchangeability of the beneficial effects 
of crossing and of new conditions, serves to explain some very 
puzzling phenomena in the forms and economy of flowers. 
Remarks on the Facts of Ihjbridity. 
The facts that have now been adduced, though not very 
numerous, are sufficiently conclusive to prove that the old 
belief, of the universal sterility of hybrids and fertility of 
mongrels, is incorrect. The doctrine that such a universal 
