CiiAr. IV. VARIOUS OBJECTIONS. I33 



stocked with specific forms : at the Cape of Good Hope and in 

 Australia, which support such an astonishinnf number of spe- 

 cies, many European plants have become naturalized. But 

 fj^coloiry shows us, that from an early part of the long' tertiary 

 jicriod the numljcr of species of shells, and that from the mid- 

 dle part of this same period the numlxT of mammals, has not 

 greatly or at all increased. AVhat, tluMi, checks an indefinite 

 increase in the number of species? The amount of life (I do 

 ncjt mean the lunnbcr of specific forms) supported on any area 

 nuist have a limit, depending;' so larj^ely as it does on physical 

 conditions ; therefore, if an area be inhabited by very many 

 species, each or neaily each s})ecies "will be represented by few 

 individuals; and such species will be liable to extermination 

 fiom accidental lluctuations in the nature of the seasons or in 

 the number of their enemies. The process of extermination in 

 these <;!ases will be ra]>id, whereas the production of new spe- 

 cies must alwa3'S be slow. Imac^inc the extreme case of as 

 many species as individuals in England, and the first severe 

 winter or A-ery dry summer would exterminate thousands on 

 thousands of species. Kare species, and each species will be- 

 come rare if the number of species in any country becomes 

 indefinitely increased, will, on the principle often explained, 

 j)resent within a given period few favorable variations; conse- 

 <|uently, the process of giving birth to new specific forms will 

 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 Ked Deer in Scotland, and of 

 liears in Norway, etc. Lastly, and this I am inclined to think 

 is the most important element, a dominant species, Avhich has 

 nlnMdy beaten many competitors in its own home, Avill tend to 

 spread and supplant many others. Alph. de Candolle has 

 shown that those species which spread widely tend generally to 

 spread vcnj widely ; and, consequently, they will tend to sup- 

 |)lMnt and exterminate 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 south- 

 east corner of Australia, where, apparently, there are manv in- 

 vaders from difTerent quarters of the world, the endemic Aus- 

 tralian species have been greatly reduced in number. ITow 

 nuich weight to attribute to these several considerations I do 

 not jiretend to assign ; but ronjointly they must limit in each 

 country the tendency to an indefinite augmentation of specific 

 fonns. 



