SUMMARY OF CHAPTER 141 



specific forms throughout the world. Dr. Hooker has re- 

 cently shown that in the S.E. corner of Australia, where, 

 apparently, 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 

 individual 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 dis- 

 puted; then, considering the infinite complexity of the rela- 

 tions of all organic beings to each other and to their condi- 

 tions of life, causing an infinite diversity in structure, consti- 

 tution, 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 

 individuals 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 improvement 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 

 organism. Nevertheless, low and simple forms will long 

 endure if well fitted for their simple conditions of life. 



Natural selection, on the principle of qualities being in- 

 herited 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 



