380 COOK 



linic or clonic individuals, which no longer share the symbasic 

 interbreeding of the group and afford no true criterion of the 

 conditions under which evolution goes forward. Just as most 

 planets are attended by satellites, so species are sometimes 

 found to be supplemented by small subspecific adjuncts, little 

 species-like groups of organisms which some have taken for 

 new or incipient species, but which stand in a permanently sub- 

 ordinate or retrograde relation to the evolutionary part of the 

 species. 



LIMITATIONS OF CLONIC PROPAGATION. 



Vegetative propagation, whether in nature or in domestication, 

 appears to conduce always to seedlessness. Some have thought 

 to explain this fact by reference to the superiority of the asexual 

 over the sexual propagation. This reasoning is scarcely ade- 

 quate, in view of the fact that much larger numbers of species 

 have retained their capacity of producing seeds, though regu- 

 larly supplementing the sexual by the vegetative propagation. 

 The greater probability is that the decline of sexual fertility in 

 vegetatively propagated types is a symptom of deterioration, 

 just as sterility is a frequent characteristic of abnormal vari- 

 ations or of hybrids. 



The formation of the sex-cells, as we now know, is a highly 

 specialized and complicated process, and it is easy to understand 

 why it should be the first of the physiological functions to 

 become deranged and inefficient. It is known also, from the 

 behavior of hybrids and mutations, that vegetative vigor has no 

 direct relation or apparent connection with reproductive vigor. 

 Indeed, sterile hybrids and mutations often show great and 

 notably superior strength and longevity, due, we may suppose, 

 to the stimulation which attends new variations. This con- 

 sideration may also explain why clonic and linic species usually 

 appear to consist of definite groups of closely similar individuals. 

 These groups may have originated by individual mutative vari- 

 ations of notable vegetative vigor, which have on this account 

 survived or crowded out the weakening survivors of the original 

 symbasic species or other variations less recent or less vigorous. 



The disastrous effects of inbreeding among the higher ani- 

 mals have been known for centuries, and are taken into account 



