326 COOK 



duced sideways. The doctrine of selection, like that of muta- 

 tion, looks upon lateral or transverse displacements as the steps 

 by which evolution is accomplished. From the kinetic stand- 

 point it appears obvious that only those lateral movements really 

 contribute to the evolution of the species which make a lasting 

 addition to the internal diversity of the species and broaden and 

 strengthen the structural network of descent. Mutations which 

 arise under conditions of inbreeding do not serve this purpose. 

 They are loose loups or free ends of the fabric of descent, 

 torn out by the disarrangement of the tensions of the specific 

 machinery of development. They do not affect the species, of 

 course, if they remain isolated from it. On the other hand, 

 mutations which are allowed to interbreed freely with the wild 

 type, or even with each other, loose their distinctive peculiarities 

 and are merged back toward the ancestral form, and [toward 

 the more normal condition of promiscuous individual diversity. 



As evolutionary phenomena the mutations described by Pro- 

 fessor De Vries have not less of interest and significance than 

 the facts of adaptation and environmental adjustment which 

 served as the basis of earlier theories of evolution. And like 

 the data of the earlier theories, the facts of mutation are capable 

 of being interpreted in a very different relation to the evolution- 

 ary motion of specific groups of organisms. Since constructive 

 evolution is accomplished, as far as we know, only in these 

 large groups of freely interbreeding individuals, we may well 

 be cautious in the acceptance of any doctrines which do not 

 take into account the normal constitution of species, and the 

 nature of the motion by which their evolutionary progress is 

 accomplished. 



A species is not a merely arbitrary collection or aggregate of 

 organisms ; it is itself an organization by which organic exist- 

 ence is maintained and organic evolution is accomplished. It 

 is customary to think of the higher types of organisms as hav- 

 ing been made possible by the association of greater and greater 

 numbers of cells, but this association and specialization of cells 

 into tissues and organs has not been accomplished without the 

 meeting of another evolutionary requirement, the association of 

 the organisms into large interbreeding groups, or species. 



