84 Cook Hybrids and Mutations. 



evolutionary significance, but the degenerative character of or 

 ganisms which have suffered such abnormally abrupt changes 

 is rendered obvious by their inability to propagate their kind. 

 The partial or complete sterility, both of hybrids and of 

 " sports " or " mutations," as the variations of inbred plants are 

 now called, has long been a matter of common knowledge among 

 breeders of plants and animals, but current evolutionary theories 

 do not associate the two groups of phenomena as belonging to 

 corresponding sidepaths of the evolutionary thoroughfare. The 

 failure to recognize this relationship is to be explained partly 

 by the general carelessness in applying such terms as " hybrid " 

 to a great variety of evolutionary conditions,* and partly by the 

 fact that in spite of their declining reproductive power, both 

 mutations and hybrids often show striking vegetative vigor. 



ECONOMIC VALUE VERSUS REPRODUCTIVE FERTILITY. 



To recognize and, if possible, to account for this paradox is of 

 practical as well as of theoretical importance, since the propa 

 gator, like the biologist, commonly reasons that the more rapid 

 and vigorous the growth of the young plant, the earlier and the 

 larger the harvest. Indeed, this calculation is generally correct, 

 since a large proportion of our domesticated species are not 

 valued for their reproductive efficiency, but for one or another 

 of their vegetative parts. Even in our horticultural crops, such 

 as apples, pears, cherries, plums, berries, oranges, pineapples, and 

 bananas, which we think of as being planted for their fruits, it 

 is not the seed itself which is utilized or desired, but the fleshy 

 pulp. The decline of reproductive fertility, or tendency toward 

 seedlessness, is not looked upon as a disadvantage, if the plant 

 can be propagated asexually, but often lends special value to a 

 new variety, particularly if correlated with vegetative vigor. 



The great economic value of a seedless grape or orange need 

 not obscure, however, the obvious fact that the plant itself is 

 degenerate, and would have no prospect of self-perpetuation 

 under natural conditions.f Neither should the utility of some 



* Popular Science Monthly, 63 : 225, 1903. 



f Mr. Walter T. Swingle notes that in some of the asexually propagated 

 cacti of Arizona vegetative vigor might more than compensate for seed 

 lessness, so that nearly sterile hybrids or mutations would have a distinct 

 advantage over the parental types. 



