REVIEWS. 



467 



season of the year, during each generation or at intervals it has to struggle for 

 life, and to sniffer great destruction. When we reflect on this struggle we may 

 console ourselves with the full beUef, 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. 



Secondly, — There is in nature a principle of natural selection. — "How will the 

 struggle for existence," says Mr. Darwin, " discussed too briefly in the last 

 chapter, act with regard to variation ? Can the principles of selection, which 

 we have seen so potent in the hands of man, apply in nature ? I think that we 

 shall see that it can most effectually. Let it be borne in mind in what endless 

 number of strange pecidiarities our domestic productions, and in a lesser degree, 

 those under nature vary ; and how strong the hereditary tendency is. Under 

 domestication, it may be truly 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 occuiTcd that other variations useful 

 in some way to each being in the great and complex battle of life should some- 

 times occur 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 sui-viving and of procreating their kind? On the 

 other hand we may feel sure that any variation in the least degree injurious 

 I would be rigidly destroyed. This preservation of favourable variations and 

 I the rejection of injurious variations I call natural selection. Variations neither 

 useful nor mjnrious 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 coui'se of natiu'al selection by taking the 

 case of a country undergomg 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 Avhich the inhabitants 

 of each country are bomid 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 certauily immigrate, and this also would 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 filled 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 course 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 Hfe 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 selection can do nothing. 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 differences — so could 



