242 EVOLUTION IN NATURE AND UNDER DOMESTICATION. 



its, all influence the possibility of evolution, but it is evident 

 that the most important division of all organisms in respect 

 to their chances of evolution is that between organisms which 

 cross, and organisms which do not habitually cross. 



In organisms of the latter category, any individual is a po- 

 tential species. If its genotype insures it a good chance of life, 

 its descendants are absolutely isolated from other lines of organ- 

 isms. One of the three conditions for the origin of new species 

 is therefore in these organisms fulfilled under any circumstan- 

 ce and in any environment. Another fundamental difference 

 between organisms of this group and crossing organisms is, 

 that the potential variability of every species is very soon zero. 

 Therefore one species cannot have daughter-species which are 

 directly and exclusively derived from it. The potential vari- 

 ability which are directly and exclusively derived from it. The 

 potential variability necessary for the production of species 

 with new genotype is possessed only by hybrids. In other words, 

 species in this group necessarily have a poly-genetic origin, 

 whereas in allogamous animals and plants at least the possibil- 

 ity exists of the origin of a new species from an original one. 



Groups of organisms which are quite closely related, may 

 yet differ in this respect, so that the mode of evolution in the 

 one differs fundamentally from that in the other group. Wheat 

 for instance, is almost exclusively autogamous, and new species 

 in this plant can arise only from hybridizations between spe- 

 cies. On the other hand in this group, almost any individual 

 plant is a potential species. Rye crosses freely, and in respect 

 to evolution it is fundamentally different from wheat. The 

 origin of new species in rye is closely similar to that in animals, 

 and yet, wheat and rye are similar enough to admit of crosses 

 between them. 



In the evolution of species of those organisms which do 

 not habitually cross, isolation is provided for, so that, when- 

 ever there is a heightening of the potential variability as the re- 

 sult of a cross, numerous new species can originate. In the estab- 

 lishment of those, the actual fitness of their genotype decides 



