186 INBEEEDING AND OUTBREEDINa 



loosely linked factors, except when long periods of time 

 are available and when natural elimination of undesir- 

 ables is high. 



In tracing the evolution of ideas concerning the effects 

 of inbreeding and outbreeding we must give great credit 

 to Darwin for calling attention to the importance of the 

 phenomena in relation to evolution and for being the first 

 to see that hereditary differences, rather than the mere act 

 of crossing, was the real point involved ; but with all due 

 credit to Darwin, it was not until Mendelism became 

 known, appreciated and applied that the first real attack 

 upon the problem was made possible. "When linked with 

 Mendelian phenomena it was clearly recognized for the 

 first time that one and the same principle was involved in 

 the effects of inbreeding and the directly opposite effects 

 of outbreeding. Inbreeding was not a process of continual 

 degeneration. Injurious effects, if present, were due to 

 the segregation of characters. In addition to this segre- 

 gation of characters the fact was established that an in- 

 creased growth accompanied the heterozygous condition. 

 All the essential facts were accounted for. A decade later 

 the great extension of knowledge in the field of heredity 

 has made possible a still closer linking of the facts of in- 

 breeding and outbreeding with Mendelism. The hypothe- 

 sis of the complementary action of dominant factors is 

 the logical outgrowth of former views and makes the in- 

 creased growth of hybrids somewhat more understand- 

 able. The fact of a stimulation accompanying hetero- 

 zygosity is supplemented by a reason why such an effect 

 is obtained. The former view of a physiological stimula- 

 tion and the more recent conception of the combined ac- 



