CONVERGENCE OF CHARACTER. 115 



the number of species in any country becomes indefinitely 

 increased, will, on the principle often explained, present 

 within a given period few favorable variations ; conse- 

 quently, the process of giving birth to new specific forms 

 would thus be retarded. When any species becomes very 

 rare, close interbreeding will help to exterminate it; authors 

 have thought that this comes into play in accounting for the 

 deterioration of the aurochs in Lithuania, of red deer in 

 Scotland, and of bears in Norway, etc. Lastly, and this I 

 am inclined to think is the most important element, a domi- 

 nant species, which has already beaten many competitors in 

 its own home, will tend to spread and supplant many others. 

 Alph. de Candolle has shown that those species which 

 spread widely tend generally to spread very widely, conse- 

 quently they will tend to supplant and exterminate several 

 species in several areas, and thus check the inordinate in- 

 crease of specific forms throughout the world. Dr. Hooker 

 has recently shown that in the south-east 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 at- 

 tribute to these several considerations I will not pretend to 

 say; but conjointly they must limit in each country the ten- 

 dency to an indefinite augmentation of specific forms. 



SUMMARY OF CHAPTER. 



If under changing conditions of life organic beings pre- 

 sent 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 

 disputed ; then, considering the infinite 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 variations useful to any organic being ever do 

 occur, assuredly individuals thus characterized 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 characterized. This principle of 



