Cook Hybrids and Mutations. 85 



degenerate plants prevent our appreciating the worthlessness of 

 others, or keep us any longer from realizing that methods of 

 breeding calculated to increase the commercial importance of one 

 plant may be utterly destructive to another. A seedless cherry 

 might bring a fortune to its discoverer, but a vigorous and beau 

 tiful seedless coffee tree found recently in Costa Rica is of use 

 only in adding emphasis to the fact that all the known variations 

 of this plant which have appeared in cultivation are less fertile 

 than the normal type of the species, and hence are described 

 properly as degenerative, in the original, pr.ictical sense of this 

 term, and in its evolutionary sense as well. 



The evolutionary significance of the degeneracy of a large 

 proportion of the domestic varieties of plants and animals has 

 also been obscured by theories that their " improved " characters 

 have been given to them by selection. It is true that the changes 

 have taken place along with a process of selection, but nobody 

 has furnished any tangible reason for believing that the selection 

 causes the changes or can cause them. Neither has it been shown 

 that the new conditions of growth are of much evolutionary 

 significance. The important and practical difference between 

 nature and domestication seems to be that the latter implies 

 narrow inbreeding and the artificial preservation of varieties 

 which in nature would either not appear at all or which would 

 not be able to survive. 



The continued popularity of the selective theory and the con 

 sequent disregard of the degenerative character of domestic 

 varieties are due, in large measure, to the fact that so many of 

 them possess a vegetative vigor as great or greater than that of 

 the wild type of the species. A sterile hybrid, the mule,* fur 

 nishes a popular symbol of strength and hardiness, and scores 

 of similar instances might be enumerated. One of the most 

 striking is Burbank's hybrid walnut tree, which grows several 

 times as fast as either of its parents, but produces no fertile 

 seeds. 



*An authentic instance of the fertility of a female mule was encoun 

 tered last year in the vicinity of Tapachula, in the Soconusco district of 

 the State of Chiapas, Mexico. The colt was alive at birth and appar 

 ently normal, but did not survive. 



