SELECTION. 177 



isms (by the method of selective elimination) by their envir- 

 onment. 



The ultimate future of a given group of organisms, the possi- 

 bility which it has of becoming a species, depends upon the 

 equilibrium betwen the two sets of causes influencing its 

 potential variabiUty. If the situation of the group and its 

 constitution are such that the balance is in favour of a greater 

 variability, it will not be a species. If on the other hand the 

 factors, which reduce its potential variabihty, are more potent 

 than the causes for a heightening of the variabiUty of the group 

 this group should be considered a species for that reason. What 

 the contributing factors are, whose cimiulating action produces 

 one effect, or the opposite, is to a certain extent immaterial for 

 the final result. Unless for instance, slow reproduction, which 

 makes for greater potential variability than rapid reproduc- 

 tion, is off-set by geographic isolation or any other effective 

 isolation, the formation of numerous species in one territory 

 is hindered by it. A great tendency to promiscuous mating 

 may be off-set by rigorous artifical selection. We are concerned 

 in the first place with the balance of tendencies toward and 

 against reduction of the potential variability. 



It depends greatly on the nature of the material, and the 

 nature of the selection, whether this last factor will materially 

 affect the variability of a group of organisms. In some cases 

 selection acts as a means of isolation, and tends to reduce the 

 potential variability of a group in the way in which all sorts 

 of isolation must act. The most striking instances of this are 

 seen in the domestic animals and plants, where in a good many 

 instances the matings are promiscuous with the exception of 

 groups of selected individuals, which are so protected from 

 intermixing with organisms of other groups, that they consti- 

 tute species. Only relatively few of the individuals of such 

 species have off-spring which can be counted into the species, 

 only individuals with certain merits, individuals which hve up 

 to the fancier's pre-conceived ideal of the species are chosen 

 to reproduce it. Many others will eventually produce off-spring, 



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