246 COOK 



Intraspecific differences, or variations, as they have been 

 called, have been interpreted hitherto either as results of envi- 

 ronmental influences or as steps toward evolutionary change. 

 The recognition of heterism, or the diversity of normal symba- 

 sic descent, is incidental to a third explanation of the value of 

 variations, that they help to maintain the vital strength or 

 organic efficiency of the species. 



Indeed, the frequency and extent of the differences of sexes, 

 castes, races and alternating generations show not onlv that 

 organisms may change without being divided into separate 

 species, but also that diversity inside the species has an evolu- 

 tionary as well as an environmental significance. 



Heterism has, if this suggestion be well founded, a concrete 

 physiological value in the economy of the species, quite as real 

 as food and water, though of a different kind. The fuel and 

 water are necessary to keep the engine going, but it is also 

 necessary that the machine be kept in repair and from time to 

 time replaced by another built on the same plan. 



Environmental variability or power of accommodation, en- 

 ables the species to operate under a variety of external condi- 

 tions, but heteric variability provides diversity of descent, even 

 under uniform and favorable conditions, and thus makes it pos- 

 sible for the species to continue to produce new individual 

 organisms as good or better than the old. 



Theories of evolution by environmental causation have over- 

 looked heterism and have assumed that the individual members 

 of species would be alike if there were no environmental in- 

 equalities to make them different. This assumption is con- 

 trary, however, to all the pertinent facts observable in nature. 

 Acquaintance with the members of any wild species of plants 

 or animals soon shows that individual differences exist, as great, 

 and often greater, than those recognized everywhere among 

 men and women, or among horses, dogs, tulips, roses, grape- 

 vines or apple trees. Definite individual diversity, as of stature, 

 features, and thumb marks is not confined to the European races, 

 nor to the human species. Travellers newly arrived in Africa 

 or China often have the impression that the natives are all 

 closely alike, but with longer residence they appear as different 

 as Europeans. 



