Page:Popular Science Monthly Volume 63.djvu/21

Rh and other aspects, is a negative and not a positive factor in evolution. Instead of causing biological motion, environment is able only to influence its direction by presenting obstacles to some tendencies of variation while permitting others to go forward

This separation of evolution from environment is not lessened by the fact that environment frequently determines the existence or degree of expression of characters. The absence of a substance necessary to the formation of a certain color or pigment prevents its formation, as may also the absence of the heat or sunlight necessary for its elaboration. To expect that external conditions should not influence organisms would be to ignore the fact that they grow by what they take in from the outside, and can not build without materials. By being placed under different conditions two individuals can be rendered far more different than they otherwise would have been, but to call these differences 'variations' and then to generalize that variations are caused by environment is simply the old-fashioned fallacy of the undistributed middle. There is not the slightest probability that the causes which make related organisms different under different conditions are those which make organisms of common origin different under the same conditions. In his paper on 'Nutrition and Selection' Professor De Vries shows that one of the variations of the poppy depends for the degree of its manifestation upon the abundance of food, or is correlated with vegetative vigor. This does not justify, however. Professor De Vries' inference that all characters are so correlated; and that the dependence was not absolute, even in the instance described, was shown when a reversal of cultural methods did not eradicate the character. The same reasoning applied to the human species would discover that some characters appear only among well-fed people, and that such characters are hereditary and persistent, but we are not compelled on this account to infer that all the differences now existing among us have arisen through over-eating.

Unsuspected differences or powers of variation sometimes appear under new environments, but it has not been shown that such potential or latent characters are less congenital, or otherwise less normal