80 ISOLATION AS A FACTOR 



ascribe all adaptive changes and apparently no 

 others. Selection alone does not produce new 

 species, although it may continuously modify old 

 ones. Usually related species become modified 

 in parallel fashion by selection. Through adap- 

 tations to special surroundings, selection may 

 produce convergence of characters, often of such 

 a character as to give a semblance of real homol- 

 ogy. The selection of the desert gives the horned 

 toad resemblance to the cactus ; this deceives no 

 one. But it may give one cactus a deceptive 

 resemblance to another which is forced to adapt 

 itself to exactly the same conditions. 



It is not often that one species is distinguished! 

 from another by adaptive characters, or by any! 

 conceivable diiference in fitness to the same con- 

 ditions in life. In this regard all are fit, and the; 

 process of natural selection holds each one close 

 to its possible limit so long as conditions remain 

 constant. 



SOME ILLUSTRATIONS OF THE RESULTS OF 

 ISOLATION 



The formation of different breeds of sheep 

 through isolation and unconscious selection in the 

 different counties of England, as elsewhere de- 

 scribed by the writer, is apparently exactly par- 

 allel with the formation of species in nature. The 

 formation and fixing of new breeds or races 

 through conscious selection is exactly parallel 

 with this, except that in conscious artificial selec- 



