Page:The Effect of External Influences upon Development.djvu/26

22 anisotropic, anisomorphic, &c.)—dominates their whole growth. All these reactions of the organism to external influences are thus to a certain extent prearranged and provided for long in advance.

I do not mean by this that the organism cannot be affected by external influences for which it is not adapted in advance. There are numerous examples known in which unusual climatic conditions have produced changes in animals and plants. European dogs sometimes lose their hair under the influence of Indian heat, and we have here a clear proof that their organism is not adapted for the endurance of heat. A small ruddy-gold butterfly, Polyommatus phlaeas, acquires a black tinge when it comes to live in warmer climates, such as that of Southern Italy. This, again, is not to be regarded as an adaptation, but must be looked upon as a direct effect of warmth. This has been shown by Merrifield's experiments (see Note V, p. 58), the results of which agree with my own observations. In this and several similar cases there is no ground for supposing that the reaction of the scales of the butterfly is, so to speak, an intentional one—or more correctly, that the determinants of the scales were so arranged in advance by natural selection that they should produce black under the influence of a high temperature.

But in other and to all appearance similar instances the relations may be of a different nature, though at a glance it may be impossible to definitely decide that this is the case. We must at any rate be careful not to regard as necessarily accidental all the variations