ASPECTS OF KINETIC EVOLUTION 389 



mental influence or of mechanisms of heredity, both the theory 

 and practice of evolution remain mysterious and contradictory. 

 It is only after the physiological value of diversity in the con- 

 stitution of species has been recognized that we begin to gain 

 a definite appreciation of the practical bearings of evolutionary 

 facts. With nature wrongly interpreted, the results of domesti- 

 cation and breeding were likewise obscured and distorted. As 

 long as our reckoning was based on the false ideal of unifor- 

 mity and stability of species, it was not possible to gain an orderly 

 concept of even the simplest of evolutionary relations, or to 

 escape from the confusion and contraditions which have left 

 even the most concrete investigators in hopeless disaccord. 



Among breeders of plants there exists the greatest possible 

 diversity of opinion regarding the value of hybridizing as a 

 means of securing new organic forms of superior agricultural 

 utility. Some breeders have secured very valuable hybrids, 

 while others have found hybrids of no use at all as a means of 

 increasing the desirable characters of the species which they 

 were seeking to ameliorate. To explain and reconcile this 

 apparent contradiction is not only a matter of scientific interest 

 in its bearing upon the general subject of evolution ; it is also of 

 much practical importance to be able to distinguish between 

 the different kinds and combinations of subspecific groups and 

 to avoid a waste of efforts upon methods and materials which do 

 not promise useful results. 



The time has not yet come for the establishment of absolute 

 standards and criteria, if indeed such a time is ever to come. 

 There are unforseen accidents, not only in the best regulated 

 families, but in nature as well. It is the rarely unusual cir- 

 cumstance, the exception to all known rules, which may have 

 great interest and potential value. The sterility of mules is one 

 of the most invariable of the phenomena of hybridization, and 

 yet fertile mules are not altogether unknown, nor is it certain 

 that such an animal might not be a means of securing new and 

 desirable variations of our equine stocks. Hybrids between the 

 different species of bovine animals are generally fertile and 

 readily made, but the establishment of a breed combining the 

 blood of the buffalo and the domestic cow has proved difficult. 



