32 THEORY OF ORGANIC EVOLUTION. 



remotely related varieties. Every other distinguish- 

 ing characteristic is wanting. 



Races arise from gamogenic or pathological 

 variations of the idioplasm. In the former case they 

 presuppose crossing between related varieties or 

 species, in the latter case an increased sensibility 

 and weakening of the idioplasm. Very often both 

 causes co-operate, since crossing follows more easily 

 when the idioplasm is weakened by hurtful influ- 

 ences and since the irritability and weakening of 

 the idioplasm increases if crossing has preceded. 

 Race formation begins in single individuals. Among 

 several individuals it begins in various directions 

 because the causes are different and hence may dis- 

 play a great multiformity. Races are distinguished 

 by more or less abnormal characteristics; they arise 

 quickly often in a single generation and present 

 various degrees of stability. This stability is insured 

 to some extent only by the strictest in-and-in breed- 

 ing. All races disappear through crossing, likewise 

 many races that have arisen from pathological 

 variations disappear even in sexual reproduction 

 (in self- fecundation). Races belong exclusively to 

 cultivation, where they can develop and exist pro- 

 tected from free intercrossing. 



While varieties and races arise by progressional 

 or stationary variation of the idioplasm, modifica- 

 tions are produced by such influences of nutrition 

 and climate as act only on the soma-plasm and the 



