158 EVOLUTION 



tion are great variability, large numbers of 

 individuals, the complex effects of inter- 

 crossing, isolation in confined areas (yet 

 probably still more an extension over con- 

 tinental areas, especially if oscillating in 

 level), and considerable lapse of time. But 

 the lapse of time by itself must not be sup- 

 posed to do anything (as if the forms of life 

 were undergoing change by some innate 

 law), but merely to afford increased oppor- 

 tunity for variation and environmental 

 change. Extinction, to which rare species 

 are on the way, is the last word of natural 

 selection. 



The divergence of character brought about 

 by artificial selection in domestic breeds is 

 efficiently paralleled in Nature, since the 

 more diversified the offspring of each species, 

 the more they will seize on diverse places in 

 the economy of Nature, and so increase in 

 numbers. The greatest amount of life can 

 be supported by increased diversification of 

 structure, each species being adapted to a 

 particular set of conditions. This divergence 

 of character, with extinction of intermediate 

 forms, explains the difficulties of classification 

 — of making a genealogical tree which will 

 express the facts of the case and represent 

 diagrammatically "the great tree of life, 

 which fills with its dead and broken branches 



