14 ORGANIC EVOLUTION — PHYSICAL 



and natural selection, and the evolution which resul 

 from the former therefore furnislies experimental (i. 

 conclusive) proof of the evolution which results fro: 

 the latter. 



The reduction towards the specific mean which inte 

 breeding tends to bring about in the qualities of ind 

 viduals as exhibited in their descendants, has cause 

 some biologists to insist that isolation is a necessaj 

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

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

 evolution proceeds not on lines of traits, however favou: 

 able, which occur infrequently or abnormally, hut on lin 

 of traits common to all the individuals of the vjholc specie 

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

 the specific average ; and bear in mind also, that general! 

 those individuals who on the whole vary fortunately i 

 regards these traits, survive and have offspring, wherej 

 generally tliose who on the whole vary unfortunate! 

 are eliminated and have no offspring, we shall have n 

 difficulty in understanding that evolution is quil 

 possible in the absence of isolation. What is nc 

 possible in the absence of isolation is evolution o 

 diverging lines. For instance, if the whole of a speci* 

 of antelope inhabited an open plain under conditior 

 common to all there might be evolution, but it woul 

 be in a direction common to the whole species. But i 

 owing to any circumstance, such as a deficiency of foo 

 supply, a portion of the species separated from the res 

 and migrated permanently to forest land, then th 

 two divisions of the species, being unable to interbreec 



1 "Without isolation, or the prevention of free intercrossin; 

 organic evolution is in no case possible. Isolation has been tl 

 universal condition of modification. Heredity and variabilit 

 being given, the whole theory of organic evolution becomes . 

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



EOMANHS. 



