OBJECTIONS TO THE THEORY OF DESCENT. 317 



principle often explained, present within a given period 

 few favorable variations ; consequently, 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 in- 

 clined to think is the most important element, a dominant 

 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 ; con- 

 sequently, they will tend to supplant and exterminate 

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

 dinate 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 greatly 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. 



THE BBOKEN BBAKCHES OF THE TEEE OF LIFE. 



Ori<nn of The affinities of all the beings of the same 



Species, class have sometimes been represented by a 

 great tree. I believe this simile largely speaks 

 the truth. The green and budding twigs may represent 

 existing species ; and those produced during former years 

 may represent the long succession of extinct species. At 

 each period of growth all the growing twigs have tried 



