HYBRID VIGOR OR HETEROSIS . 147 



of plant; in 5 out of 7 experiments, the weight was in- 

 creased. Moreover, the crossed plants frequently flow- 

 ered earlier and in many other ways showed their advan- 

 tage over the parent races. 



Darwin extended his observations to the animal king- 

 dom and his views on the whole subject are summed up 

 concisely in the following paragraph from "Animals and 

 Plants under Domestication ": "The gain in constitu- 

 tional vigor derived from an occasional cross between in- 

 dividuals of the same variety, but belonging to different 

 families, or between distinct varieties, has not been so 

 largely or so frequently discussed as have the evil effects 

 of too close interbreeding. But the former point is the 

 more important of the two, inasmuch as the evidence is 

 more decisive. The evil results from close interbreeding 

 are difficult to detect, for they accumulate slowly and 

 differ much in degree with different species, whilst the 

 good effects which almost invariably follow a cross are 

 from the first manifest. It should, however, be clearly 

 understood that the advantage of close interbreeding, as 

 far as the retention of character is concerned, is indis- 

 putable and often outweighs the evil of a slight loss of 

 constitutional vigor. " 



From this statement Darwin evidently considered the 

 ill effects of inbreeding and the good effects of crossing to 

 be two different things. He was ri.^ht in stressing the 

 benefit from crossing rather than the injury from close 

 mating, but wrong in thinking the evil effects accumulated 

 as inbreeding was continued. Such a belief is not substan- 

 tiated by more recent experiments, as has been shown in 

 the last chapter. It is true, however, that the effect of 

 inbreeding may not be as noticeable in the first generation 



