LECTURE XV. 435 



of generations^ niny produce in man, nay^ must liave produced, 

 when the savage or semi-barbarous populations knew not how 

 to protect themselves from the influences of the new climate, 

 when they had to struggle against animal and vegetable nature, 

 against the physico-chemical forces which predominated. How 

 much more destructive must at that time have been the 

 struggle for life than it is now for those travelling pioneers 

 whose courage we so much admire. And how much more 

 durable and deeper must be the traces of those struggles." 



It appears to me that many facts are here confounded which 

 ought to be separated. The struggle for existence in a new 

 country may be deadly, because the number of deaths ex- 

 ceeds that of births, and then there can be no question of a 

 change of race, for it becomes extinct ! Or, again, the struggle 

 is not so deadly, that is to say, the number of births exceeds 

 that of deaths, and the race adapts itself to the new conditions. 

 This is done within a few generations, when a condition is 

 established corresponding to the altered vital condition. We 

 have for this the clearest proofs in the domestic animals which 

 are transplanted to foreign climates. Hog, sheep, cat, and dog 

 have thus within a few generations, in southern climates, passed 

 through such changes as, for instance, the Egyptian goose in 

 Europe. After that change, which, as stated, was effected 

 within a few generations, and the race was acclimatised, no 

 further modification took place. And we may easily convince 

 ourselves that it must be so. For if modifications are requisite 

 to live in a foreign climate, these modifications must be rapidly 

 accomplished to preserve the race from threatened desti-uc- 

 tion. If then, we say, that a race which has undergone modi- 

 fications within a few generations, must for that reason undergo 

 a corresponding sum of alterations at a subsequent period ; if 

 we were to establish a quasi rule of three, and say : since tliis, 

 this race has within three generations experienced x altera- 

 tions, consequently it must within thirty generations have under- 

 gone 10 a; alterations, as Quatrefages would put it, we commit a 

 scientific error, and excite hopes which can never be realised. 



We thus infer that all instances which have been cited of 

 changes in races of pure descent, by the mere influence of 



F f2 



