CHAPTER VII 



HYBKID VIGOR OR HETEROSIS 



Whether or not inbreeding in a race of plants or ani- 

 mals results injuriously depends primarily, as we have 

 attempted to show, upon the hereditary constitution of 

 the organism. The beneficial effect of crossing, heterosis, 

 is a more widespread phenomenon. It may be expected 

 when almost all somewhat nearly rehited foniis are 

 crossed together. Even plants or animals which show no 

 harmful results of inbreeding are frequently improved 

 thus in a remarkable way. Moreover, this stimulating 

 effect is immediately apparent in the individuals result- 

 ing from the cross. It is then at its maximum. 



It is natural, therefore, that the early writers on the 

 subject should have noticed and emphasized the good to 

 be derived from crossing rather than the bad which some- 

 times results from inbreeding. Almost without exception 

 the great horticultural writers of the late eighteenth and 

 early nineteenth centuries noted the occurrence of hybrid 

 vigor, and many of them described it in great detail. 

 Among them may be mentioned Kolreuter (1763), Knight 

 (1799), Mauz (1825), Sageret (1826), Borthollot (1827), 

 Wiegmann (1828), Herbert (1837), Lecoq (1845), Giirt- 

 ner (1849). In fact, in Focke^s compilation of this early 

 work, **Die Pflanzen-Mischlinge*' (1881), cases of heter- 

 osis worthy of special mention w^ere found in fifty-nine 

 families of the flowering plants as well as in the conifers 

 and the ferns. Animal husbandmen were somewhat loss 



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