ASPECTS OF KINETIC EVOLUTION 393 



The facility with which many plants can be propagated from 

 cuttings or by grafting often permits sterile mutations and 

 crosses to be preserved and utilized for long periods of time. 

 Among animals, on the other hand, mutations are of relatively 

 small value. The higher organization of animals renders them 

 liable to earlier and more serious deterioration from inbreeding, 

 though there is great difference in the susceptibility of different 

 kinds of animals. 



BEHAVIOR OF DISCRIMINATE MUTATIONS. 



When mutations are crossed with other members of their own 

 immediate group of related individuals they are generally pre- 

 potent. They do not tend to average away and disappear, but 

 are repeated, or even accentuated, in a considerable proportion 

 of each successive generation, and sometimes in all of them. 

 Plant mutations which can be propagated by self-fertilization 

 are often constant from the first, and have been thought by some 

 to represent the formation of genuine new species. 



When mutations are bred outside of their own group, and 

 especially when they are crossed with the wild type of the 

 species or with the variety which has not been long or closely 

 selected, they are not prepotent, but recessive. The new muta- 

 tive characters appear weaker than the others and may fade out 

 and disappear entirely. The same result may be reached by 

 indiscriminate interbreeding among the representative of two or 

 more mutations or selective varieties. The ancestral characters 

 of the wild type of the species reassert themselves, and may 

 even reappear in crosses between varieties from which they 

 have long been lost. 



All these and other similar phenomena can be understood, 

 or at least brought into rational relations, if we keep in mind 

 the fact that crosses between the narrowly selected varieties or 

 mutations of the same species tend to restore the original and 

 normal conditions of free interbreeding. They tend, in other 

 words, to repair and reconstruct the normal fabric of symbasic 

 descent, and to reduce the strains and deteriorations caused by too 

 close segregation, too little diversity, and too much inbreeding. 



Instead of being monstrous or unnatural, these crosses are 



