72 A NEW THEORY OF EVOLUTION. 



his theory, but, assuming the existence of 

 life in its simplest forms, sets forth at great 

 length the facts and arguments, which he 

 says have thoroughly convinced him 

 That all animals and plants have been 

 evolved from a few progenitors, and 

 possibly from one prototype only. 

 That new species have been evolved 

 principally by natural selection, acting 

 through the struggle for existence ; 

 aided (a) to an important extent by the 

 inherited effect of the use and disuse 

 of parts, and (b) to an unimportant 

 extent by the direct action of the ex- 

 ternal conditions of life. 

 That small beneficial differences appeared 

 from time to time in some members 

 of a species, and, being accumulated 

 by heredity through numberless suc- 

 cessive generations, during vastly ex- 

 tended periods of time, ultimately be- 

 came specific variations, and thus new 

 species arose. 1 



The principal phenomena adduced in sup- 

 port of these conclusions are 



The similarity in structure and organisa- 

 tion of successive types. 



1 See 'Origin of Species,' Ed. vi., p. 421. 



