REVIEWS. 
467 
season of the year, during each f^cncraiion or at intervals it has to struggle for 
life, and to siiH'cr great destruction. When we reflect on this struggle we may 
console ourselves with the full belief, that the war of nature is not incessant — 
that no fear is felt — that death is generally prompt, — and that the vigorous, the 
healthy and the happy, survive and multiply. 
Seco/idli/, — There is in nature a jrrincijde of natural selection. — "How will the 
struggle for existence," says Mr. Darwin, " discussed too briefly in the last 
chapter, act with regard to variation P Can the principles of selection, which 
we have seen so potent in the hands of man, apply in natui'e ? I thmk that wc 
shall sec tiiat it can most etfcctnally. Let it be borne iu mind in what endless 
number of strange peculiarities oiu' domestic productions, and in a lesser degree, 
those under natiu-e vary ; and how strong the hereditary tendency is. Under 
domestication, it may be trrdy said that the whole organization becomes in some 
degree plastic. Let it be borne in mind how infinitely complex, and close- 
fitting are the mutual relations of all organic beings to each other and to their 
physical conditions of life. Can it, then, be thought improbable, seeing that 
variations useful to man have undoubtedly oecniTed that other variations useful 
in some way to each being in the great and complex battle of life should some- 
times oecnr in the course of thousands of generations ? If such do occur, can 
we doubt (remembering that many more individuals are born than can possibly 
survive), that individuals having advantages however slight, over others, would 
have the best chance of surviving and of procreating their kind? On the 
other hand we may feel sui'e that any variation in the least degree injurious 
would be rigidly destroyed. This preservation of favourable variations and 
the rejection of injurious variations I call natui-al selection. Variations neither 
useful nor injurious would not be affected by natural selection and would be 
left a fluctuating element, as perhaps we see in the species called polymorphic. 
We shall best understand the probable course of natural selection by taking the 
case of a country undergoing some physical change, for instance, of climate. 
The proportional numbers of its inhabitants would almost immediately undergo 
a change, and some species might become extinct. We may conclude from 
what we have seen of the intimate and complex manner in which the inhabitants 
of each country are bound together, that any change in the numerical proportions 
of some of the inhabitants, independently of the change of climate itself, would 
seriously affect the others. If the country were open at its borders, new forms 
would certainly immigrate, and this also woidd seriously disturb the relations 
of some of the former inhabitants. Let it be remembered how powerful the 
influence of a single introduced tree or mammal has been shewn to be. But in 
the case of an island, or of a country partly surrounded by barriers, into which 
new and better adapted forms could not freely enter, we should then have places 
in the economy of nature which would decidedly be better fiUed up, if some of 
the original inhabitants were in some manner modified ; and had the area been 
open to immigration, these same places would have been seized on by intruders. 
In such case, every slight modification, which, in the coui-se of ages chanced to 
arise, and which in any way favoured the individuals of any species, by better 
adapting them to their altered conditions, would tend to be preserved, and 
natural selection would thus have free scope for the work of improvement. 
We have reason to believe, that a change in the conditions of life by specially 
acting on the reproductive system, causes or increases variability ; and in the 
foregoing case the conditions of life are supposed to have undergone a change 
and this would manifestly be favourable to natural selection, by giving a better 
chance of profitable variations occurring, and unless profitable variations do 
occur, natural selcetiou can do nothmg. Not that, as I believe, any extreme 
amount of variability is necessary ; as man can certainly produce great results 
by adding up in any given direction mere individual oifferences — so could 
