Cook — Hybrids and Mutations. 89 



too violent, and its vigor may be increased even b}' tlie degen- 

 erative variations wliich follow u})on the absence of normal inter- 

 breeding. When thus halted or hindered the vital mechanism 

 l)ut 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 vidiich appear under narrow inbreeding and wide 

 cross-breeding are abnormal in their amplitude, like fluctuations 

 of temperature in disease. That even comi)letely sterile muta- 

 tions and hybrids may enjoy exceptional vigor does not change 

 the fact of abnormalit}^ 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 apparentl}' normal blossoms in profusion, but 

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

 be produced which will not germinate, or seedlings maj^ grow, 

 but never mature. There are all possible stages from normal 

 fertilit}' to complete sterilit}^ as there are endless gradations 

 between normal shape and monstrous deformity. 



The present interpretation of the facts has at least the merit 

 of simiilicit}^ 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 evolutionar}- 

 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. 



Ph3'siolog3^ 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 

 ph3'siological process, since it aff"ects not merel}' 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. 



