Page:Popular Science Monthly Volume 77.djvu/31

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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 particular 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 variation 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 individual, 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 variation, 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 supposing 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