96 NATUEAL SELECTION. [CHAP. IV. 



there are many invaders from different quarters of the globe, the 

 endemic Australian species have been greatly reduced in number. 

 How much weight to attribute to these several considerations 

 I will not pretend to say; but conjointly they must limit in each 

 country the tendency to an indefinite augmentation of specific 

 forms. 



Summary of Chapter. 



If under changing conditions of life organic beings present indi 

 vidual differences in almost every part of their structure, and this 

 cannot be disputed ; if there be, owing to their geometrical rate 

 of increase, a severe struggle for life at some age, season, or year, 

 and this certainly cannot be disputed ; then, considering the in- 

 finite complexity of the relations of all organic beings to each 

 other and to their conditions of life, causing an infinite diversity 

 in structure, constitution, and habits, to be advantageous to them, 

 it would be a most extraordinary fact if no variations had ever 

 occurred useful to each being's own welfare, in the same manner 

 as so many variations have occurred useful to man. But if varia- 

 tions useful to any organic being ever do occur, assuredly indivi- 

 duals thus characterised will have the best chance of being 

 preserved in the struggle for life ; and from the strong principle 

 of inheritance, these will tend to produce offspring similarly 

 characterised. This principle of preservation, or the survival of 

 the fittest, I have called Natural Selection. It leads to the im- 

 provement of each creature in relation to its organic and inorganic 

 conditions of life ; and consequently, in most cases, to what must 

 be regarded as an advance in organisation. Nevertheless, low and 

 simple forms will long endure if well fitted for their simple con- 

 ditions of life. 



Natural selection, on the principle of qualities being inherited 

 at corresponding ages, can modify the egg, seed, or young, as 

 easily as the adult. Amongst many animals, sexual selection will 

 have given its aid to ordinary selection, by assuring to the most 

 vigorous and best adapted males the greatest number of offspring. 

 Sexual selection will also give characters useful to the males 

 alone, in their struggles or rivalry with other males ; and these 

 characters will be transmitted to one sex or to both sexes, accord- 

 ing to the form of inheritance which prevails. 



Whether natural selection has really thus acted in 'adapting the 

 various forms of life to their several conditions and stations, must 

 be judged by the general tenor and balance of evidence given in 

 the following chapters. But we have already seen how it entails 

 extinction; and how largely extinction has acted in the world's 

 history, geology plainly declares. Natural selection, also, leada 

 to divergence of character ; for the more organic beings diverge in 





