FACTORS OP ORGANIC EVOLUTION. 461 



innate attribute of life. "But we must, I think, conclude" * * * 

 writes Darwiu in tlie latter book, " that organic beings, when subjected 

 during several generations to any change whatever in their conditions, 

 tend to vary." He discussed at length the particular agencies which 

 he considered to be most potent in inducing variability, and enumerated, 

 amongst other factors the kind and amount of food, climate, and cross- 

 ing. ''Changes of any kind in the conditions of life," he repeats, 

 "even extremely slight changes, often suffice to cause variability. 

 Excess of nutriment is perhaps the most efficient single exciting" cause." 

 Cope, in his discussion of the "Causes of variation," starts out with 

 the proposition "to cite examples of the direct modifying effect of 

 external influences on the characters of individual animals and plants," 

 and he closes with this paragraph: "I trust that I have adduced evi- 

 dence to show that the stimuli of chemical and physical forces, and also 

 molar motion or use and its absence, are abundantly sufficient to pro- 

 duce variations of all kinds in organic beings. The variations may be 

 in color, proportions, or details of structure, according to the conditions 

 which are present." This is, in great part, the thesis to which Darwin 

 extended the proofs of a most laborious collection of data from garden- 

 ers and stock-breeders and from feral nature. It has been the great 

 misfortune of the interpretation of Darwin's writings that his hypoth- 

 esis of natural selection has so completely overtopped everything else 

 in the reader's mind that other important matters have been overlooked. 

 While the one central truth in the plant creation is the fact that 

 differences arise as a result of variations in environment, there are 

 nevertheless many exceptions to it. There are various types of differ- 

 ences which are merely incidental or secondary to the main stem of 

 adaptive ascent. Some of these are such as arise from the cessation 

 of the constructive agencies, and others are mere correlatives or accom- 

 paniment of type differences. As an example of the former, we may 

 cite the behavior of the potato. By high cultivation and careful breed- 

 ing, the plant has been developed to produce enormous crops of very 

 large tubers, so heavy a crop that the plant has been obliged to spare 

 some of its energy from the production of x)ollen and berries for the 

 purpose of maintaining the subterranean product. It is evident that 

 this high state of amelioration can be maintained only by means of 

 high cultivation. The moment there is a let-down in the factors which 

 have bred and maintained the plant, there is a tendency toward a 

 breaking up and disappearance of the high bred type. This is an illus- 

 tration of the phenomenon of panmixia, as outlined by Weismann, except 

 that the force which has ceased to act is human selection rather than 

 natural selection. "This suspension of the preserving influence of 

 natural selection," Weismann writes, " may be termed Panmixia." In 

 his opinion, "the greater number of those variations which are usually 

 attributed to the direct influence of external conditions of life are to 

 be attributed to panmixia. For examx)le, the great variability of most 



