20 POPULAR SCIENCE MONTHLY. 



and kinetic theories of evolution. Under the former mutations have 

 been accepted as genuine examples of the methods by which species 

 are formed in nature, while under the latter they appear as but the 

 dying spasms of small groups of organisms suffering a fatal separation 

 from the life of their species. 



Mr. A. F. Woods has kindly brought to my attention an important 

 confirmation of this association of mutation with reproductive debility, 

 namely, that cultural methods calculated to encourage vegetative growth 

 at the expense of reproductive vigor or fertility are also distinctly 

 favorable to the appearance of mutations and of physiological abnor- 

 malities such as variegation of foliage. Professor De Vries made Oeno- 

 thera the special object of his study because the frequency of fasciation 

 and other monstrosities seemed to indicate a high degree of structural 

 instability. The abnormality of this class of evolutionary phenomena 

 was not considered. It was inferred instead that the condition of ' muta- 

 tion' is a somewhat rare and temporary state through which organisms 

 pass at the period of formation of new species, and the failure to find 

 equal 'mutability' in other plants did not prevent the drawing of gen- 

 eral conclusions. 



Definitions of Evolutionary Stages. 



As a summary of the above discussion three evolutionary condi- 

 tions may be formally distinguished: 



1. Prostholytic or Progressive Stage. — The prostholytic or progres- 

 sive stage of evolution is found in large species of wide distribution 

 containing abundant individuals with free intercrossing of numerous 

 lines of descent. There is unlimited diversity or inconstancy of in- 

 dividual characters, and variation is indefinite and continuous in the 

 sense that endless fluctuations and intergradations are present. The 

 requirements of symbasis are fully met; interbreeding is normal and 

 reproductive fertility is high. 



2. Hemilytic or Retarded Stage. — The hemilytic or retarded stage 

 of evolution is reached in species or subordinate groups of restricted 

 distribution containing a limited number of individuals with few and 

 closely interrelated lines of descent. Characters are nearly uniform 

 and variation slight. The requirements of symbasis are not fully met, 

 but the deficiency has not yet resulted in reproductive debility. 



3. Catalytic or Declining Stage. — The catalytic or declining stage 

 of evolution appears in closely segregated groups of relatively few indi- 

 viduals propagated by inbreeding or on single lines of descent. Varia- 

 tions are few, pronounced, and abrupt or discontinuous, also relatively 

 constant and with little or no intergradation. The catalytic stage 

 implies a violation of the law of symbasis, or inadequate cross-fertiliza- 

 tion, together with the resulting deficiency of fertility. 



