14 ORGANIC EVOLUTION — PHYSICAL 



aiul natural sclcctiou, and the evolution which results 

 from the former therefore furnishes experimental (i. e. 

 conclusive) proof of the evolution which results from 

 the latter. 



The reduction towards the specific mean which inter- 

 breeding tends to bring about in the qualities of indi- 

 viduals as exhibited in their descendants, has caused 

 some biologists to insist that isolation is a necessary 

 antecedent to evolution.^ But if we bear in mind the 

 fact (to be more fully discussed in a future page) that 

 evolution proceeds oiot on lines of traits, liuivcvcr favour- 

 able, wldcli occur infrequently or abnormally, hut on lines 

 of traits common to all the individuals of the vjholc species, 

 that is, in which every individual rises above or falls below 

 the specific average ; and bear in mind also, that generally 

 those individuals Avho on the whole vary fortunately as 

 regards these traits, survive and have offspring, whereas 

 generally those who on the whole vary unfortunately 

 are eliminated and have no offspring, we shall have no 

 difficulty in understanding that evolution is quite 

 possible in the absence of isolation. What is not 

 possible in the absence of isolation is evolution on 

 diverging lines. For instance, if the whole of a species 

 of antelope inhabited an open plain under conditions 

 common to all there might be evolution, but it would 

 be in a direction common to the whole species. But if, 

 owing to any circumstance, such as a deficiency of food 

 supply, a portion of the species separated from the rest, 

 and migrated permanently to forest land, then the 

 two divisions of the species, being unable to interbreed. 



6> 



1 " Without isolation, or the prevention of free intercrossing, 

 organic evolution is in no case possible. Isohition lias been the 

 universal condition of modification. Heredity and variability- 

 being given, the whole theory of organic evolution becomes a 

 theory of the causes and conditions which lead to isolation "— 

 Romanes. 



