HEREDITY VERSUS ENVIRONMENT

THIS is a convenient place to deal with the layman’s perennial question, ‘Which is the more important, Heredity or Environment?’ So general a question is irrelevant; the biologist’s answer is on these lines : A living thing starts individual existence as an egg-cell—a tiny packet of chemicals, and the precise amount and nature of those chemicals decides whether it shall grow up a frog or a snapdragon, a fly or a man. That is heredity. But it would never grow up at .ill unless it had the appropriate environment.

The lesser differences between a pink and a yellow snapdragon, a cart-horse and a race-horse, a negro and a white man are just as much due to heredity as the larger specific differences. The general question about heredity or environment therefore breaks up into a hundred little questions of detail, such as :

Does the seed or the soil determine a good wheat crop? The answer to this is : ‘Both.’ Does heredity or environment make one frog bright yellow and its brother dark greenish-brown?—Environment (temperature and moisture).

Does heredity or environment make one man energetic and able, another a comatose mental defective?—Heredity (bar a few cases caused by accident or disease before or shortly after birth).

Each character presents its own set of problems, and there can be no general answer. But there are certain general methods of investigating all of them, and the simplest of these is to find out whether a character occurs more often or more fully after inbreeding. If it does, then you can feel fairly certain it is hereditary in the strictest sense—like eye-colour in man—since inbreeding intensifies all the hereditary qualities.

If the strength of a character varies in brothers and sisters (like the skin-colour of the mulattoes’ children) then, again, it is probably hereditary, since brothers and sisters have much the same environment, but not, thanks to the chromosome

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dance, the same heredity. If a character suddenly crops up ‘out of the blue,’ the odds are in favour of heredity, that it is a deep-buried recessive that has at last come to the surface. But if the character you are investigating appears co?isis-tently in all the members of a family, or only varies step by step with the circumstances, then you must suspect the preponderance of environmental influence. The chief scientific trouble, though, is not so much disentangling heredity and environment, as defining and measuring ‘quantitative characters,’ as they are called, such as intelligence, vigour, disease-resistance. The attempts to do this are discussed in the books mentioned at the end.

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