86 THE EVOLUTION OF LIVING BEINGS. 



majority being in F 2 of a cross of two forms differing 

 in one respect only, 3 : 1. If, as in rabbits, the domi- 

 nant colour f. i. is selected for the protective effect 

 it confers on the animals possessing it, this majority 

 of course increases rapidly. 



We can imagine however easily, that a change of 

 conditions f. i. a migration of the rabbits from the 

 dunes to black earth, would divest the dominant color, 

 grey, from its protective power, and thus cause an 

 increase of the, now protectively colored, black reces- 

 cives. 



This might even go so far that only one type, reces- 

 sive in all characters, survives in which case the Lin- 

 neon has been reduced to a pure species and conse- 

 quently stops to „vary." 



As „ variation" by crossing is — by the plasticity it 

 confers on the Linneon — a means and probably 

 the chief one, to adapt such a Linneon to changing 

 conditions the dying out of certain Linneons may 

 have been caused by such a survival of pure recessi- 

 ves only, putting a final stop to the possibility of 

 such a species to adapt itself to changed conditions, 

 unless a cross with an individual belonging to another 

 Linneon, creates fresh opportunities to „vary." 



If no such cross occurs, if such a Linneon reduced 

 to a pure species is doomed to remain single, it is — if 

 conditions change — unable to form a progeny able 

 to adapt itself to these changed conditions, and conse- 

 quently must die out, exactly as human families 

 whose members remain single; though it may remain 

 in existence for a considerable time, if no untoward 



