THE FUTURE OF THE 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 
