122 SUMMARY. 



each species will be represented by few individuals; and 

 such species will be liable to extermination from accidental 

 fluctuations in the nature of the seasons or in the number 

 of their enemies. The process of extermination in such 

 cases would be rapid, whereas the production of new species 

 must always be slow. Imao^ine the extreme case of as manv 

 Species as individuals in England, and the first severe winter 

 or very dry summer would exterminate thousands on thou- 

 sands of species. Eare species, and each species will become 

 rare if the number of species in any country becomes in- 

 definitely increased, will, on the principle often explained, 

 present within a given period few favorable variations; con- 

 sequently, the process of giving birth to new specific forms 

 would thus be retarded. AYhen 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 

 dominant species, which has already beaten many compet- 

 itors 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 veri/ 

 widely, consequently they will tend to supplant and exter- 

 minate several species in several areas, and thus check the 

 inordinate increase of specific forms throughout the world. 

 Dr. Hooker has recently shown that in the southeast cor- 

 ner of Australia, where, apparently, there are many in- 

 vaders from different quarters of the globe, the endemic 

 Australian species have been greath^ reduced in number. 

 How much weight to attribute to these several considera- 

 tions I will not pretend to say; but conjointly they must 

 limit in each country the tendency to an indefinite aug- 

 mentation of specific forms. 



SUMMARY OF CHAPTEB. 



If under clianging 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 cannoi 



