ASPECTS OF KINETIC EVOLUTION 359 



of constitution, the specific. A species is quite as concrete a 

 phenomenon as a crystal. Both are collections or aggregates 

 of smaller units, and the units have in both cases definite and 

 necessary relations to each other on which the existence and 

 further development of the crystal or the species depend. 



It is true that many valuable evolutionary data have been 

 secured from captive or domesticated plants and animals, but 

 the results of this whole class of experiments indicate very 

 definitely that evolutionary phenomena under these conditions 

 are degenerative and not constructive. We are driven back to 

 study the constitution of species in nature, to gain a clear under- 

 standing of the organic conditions which make possible genuine 

 developmental progress, a true organic evolution. 



No theory or evolutionary interpretation can hope for per- 

 manence which leaves out of account this primary fact that 

 organisms normally exist in large groups of freely interbreeding 

 individuals, the groups commonly called species. Domesticated 

 varieties of plants exist without interbreeding and a few species 

 in nature are supposed to propagate only by vegetative methods, 

 by parthenogenesis or by self-fertilization, but no genus, family 

 or order appears ever to have developed without the association 

 of the individual organisms into interbreeding groups or species. 

 The only exceptions, if any, are among the bacteria and other 

 extremely simple forms of life which have failed to develop 

 either a specialized nuclear structure in the cells themselves or 

 an ability to associate and differentiate to form compound cellu- 

 lar organisms. 



The reigning popularity of laboratory methods of research 

 may permit small welcome for the suggestion of a method of 

 evolution which requires the extensive equipment of nature and 

 can not be demonstrated in cages or gardens, except by negative 

 results, like those already well known. This disappointment 

 need not continue, however, any longer than may be necessary 

 to perceive that while experiments with domesticated species 

 lose in apparent general significance under the new interpreta- 

 tion, they gain greatly in definiteness. If they do not show us 

 how the fabric of normal evolutionary descent is woven, they at 

 least teach us how it may be unravelled. This knowledge is of 



