Cook Hybrids and Mutations. 89 



too violent, and its vigor may be increased even by the degen 

 erative variations which follow upon the absence of normal inter 

 breeding. When thus halted or hindered the vital mechanism 

 but turns aside the further because it has lost the equilibrium 

 of normal motion. 



It is not necessary to regard variation as abnormal, but the 

 variations which appear under narrow inbreeding and wide 

 cross-breeding are abnormal in their amplitude, like fluctuations 

 of temperature in disease. That even completely sterile muta 

 tions and hybrids may enjoy exceptional vigor does not change 

 the fact of abnormality, but shows merely that the evolutionary 

 disorder affects the reproductive rather than the vegetative parts. 

 Both in hybrids and in mutations the tendency to sterility some 

 times appears so early that the plants do not produce flowers, 

 or there may be a progressive sterilization of the essential organs 

 of the flowers, as in the so-called " doubling " which has appeared 

 independently in so many mutations of cultivated plants. 

 Others may form apparently normal blossoms in profusion, but 

 set no fruits ; fruits may develop without seeds ; seeds may 

 be produced which will not germinate, or seedlings may grow, 

 but never mature. There are all possible stages from normal 

 fertility to complete sterility, as there are endless gradations 

 between normal shape and monstrous deformity. 



The present interpretation of the facts has at least the merit 

 of simplicity, since it permits us to suppose that the same evolu 

 tionary vigor appears in normal variations and crosses, and in 

 abnormal mutations and hybrids, and that the same evolutionary 

 debility affects the two latter conditions. The vigor is due nei 

 ther to sterility nor to selection, but to variation ; the sterility 

 is not explained by normal variation, nor by selection, except 

 as selection implies the absence of normal interbreeding, and 

 the consequent weakening of heredity. 



Physiology in the narrower sense, the science of nutrition and 

 other bodily functions, does not explain either the vigor or the 

 debility, but in the broader view evolution itself becomes a 

 physiological process, since it affects not merely the form and 

 structure, but determines also the quality and efficiency of the 

 organism, in quite as practical and definite a manner as do food- 

 supply and other external conditions. 



