HEREDITY IN RETICULAR DESCENT. 21 



resorting to abstract principles or to hypothetical mechanisms of hered- 

 it3\ The network of descent is, as it Avei'c, a mapshowing- the alternative 

 routes of the developing- organism, and permitting normally any combi- 

 nation of ancestral characters, as may well result from the endlessly 

 varying arrangements into which the ancestral chromatin elements may 

 fall at the time of synapsis or chromatic fusion. Twins developed from 

 the same ovum would have the same arrangement of chromatin, which 

 accords with their close similarity of form, but otherwise there is 

 unlimited diversity, even among the sinuiltaneous offspring of the 

 same parents. It would seem, therefore, that instead of a mechanism 

 of heredity inside the reproductive cells there is an automatic device 

 for insuring diversity. The higher the organization the more com- 

 plex the descent, and the greater the variet}^ of nuclear configurations 

 and the resulting individual diversit3\ 



Nevertheless, inheritance is not governed merely by chance, nor 

 limited even to the intinity of nuclear networks to be made by the 

 combinations possible among the ancestral chromatin elements. With 

 the greater vitality of interbred organisms is associated also a stronger 

 heredity or ])repotency of the wild or more ])roadly symbasic types 

 when such are crossed with inbred domesticated varieties. New varia- 

 tions, too, appear to have the same effect as diversity of descent in 

 lending greater vigor and prepotency. Even mutations, or degener- 

 ative variations induced by inbreeding, are prepotent on their own 

 plane of symbasis — that is, when crossed onl}^ with their own inbred 

 relatives — though they are promptly obliterated or " swamped" 

 when brought into contact with the broadly symbasic wild type, the 

 prepotency of the diverse descent being far greater than that attach- 

 ing to the inbred variation. It is the prepotency of variations which 

 renders evolution truly kinetic; for the methods of organic descent 

 are such as to bring about a spontaneous change of type. The envi- 

 ronment often influences the direction of this vital motion, but is in no 

 proper sense an actuating cause. '" 



Cells are the units of organization, but species, as groups of inter- 

 breeding individuals, are the units of evolution. The causes of evolu- 

 tion are not revealed by h^^pothetical subdivisions of cells into char- 

 acter units or detei'minate elements, but by ascertaining the methods 

 of descent through which interbreeding maintains organic strength 

 and evolutionary progress. Cells divide themselves, as we know, into 

 other cells, and species into other species, but it is only as cells and as 

 species that their vital, organic, evolutionary activities are accom- 

 plished. Individuals vary and mutate, but only species evolve. To 

 classify the various stages and functions of organisms under general 

 and abstract terms may be desirable, but for evolutionary purposes it 



« Cook, 0. F. Natural Selection in Kinetic Evolution, Science, n. s., 19: 549. 1904. 



