TEE FUTURE OF TEE HUMAN RACE 25 



Large and small representatives of the pure line produce progeny of the same 

 mean size. The mean size is therefore strictly hereditary throughout the pure 

 line, and it depends, not on the accidental individual dimensions of the par- 

 ticular progenitor, but on the fundamental characteristics of the pure line in 

 question. 



All this indicates that if desirable qualities, represented by units in 

 inheritance, are once obtained, and are not disturbed by crossing, they 

 may continue from generation to generation indefinitely, without varia- 

 tion other than that produced in the individual by the immediate 

 influence of the environment. 



But, here, as Professor Jennings remarks, we have to ask how the 

 different pure lines arise ? That is to say, whence the different qualities 

 which assuredly did not all coexist in the original form of life? We 

 have seen that the unit in inheritance is, to say the least, a very complex 

 object from a chemical point of view. No doubt it is easily destroyed, 

 but its usual character seems to be that of resisting molecular change 

 short of disintegration. Thus it is carried on from individual to indi- 

 vidual, virtually unaltered, or in the alternative cases, destroyed. 

 Occasionally, however, it must be subjected to some subtle influence 

 which merely disturbs its internal structure, or perhaps deprives it of 

 something it possessed. When this occurs, we have an original varia- 

 tion, the starting of something really new. Such original variations 

 must be relatively rare, and we do not know what causes usually bring 

 them about. Tower with beetles and MacDougal with plants seem to 

 have produced them, in the one case by changes of temperature and 

 moisture, in the other by chemical means. The fact that in some 

 regions certain genera produce many species, as the asters in America, 

 the brambles in Europe, seems to suggest that the disturbing influence 

 may be different for different organisms, and may be locally distributed. 

 Or it may be that, a line of disturbance once set up in some unknown 

 manner, influences prevalent anywhere are sufficient to continue the 

 line of change. 



It may be that coming generations will see the causes of original 

 variation fully elucidated, and the phenomenon itself brought largely 

 under control. While mankind would thus be furnished with a weapon 

 of extraordinary value, one trembles to think of the damage it might 

 do. It might be made the means of producing new and wonderful 

 variations in plants and animals, even in man himself; but inasmuch 

 as there is every reason to suppose that its results could not often be 

 accurately foretold, there is no telling what evil might result, even sup- 

 posing that the power was never used with intentionally malicious 

 purpose. 



We are not at present, however, in any danger of being overrun 

 with original variations; and it must be remembered that most of the 

 recent wonders of Burbank and others, which are new in a practical 



