REDUCTION OF VARIABILITY. 137 



we took it for granted that an individual, heterozygous for one 

 gene would surely mate with a homozygote, and on this as- 

 sumption our calculation of the chances against the persistence 

 of varietal characters was based. But it will have to be con- 

 ceded, that an important factor in this chance for persistence is 

 the rate of dispersal of the organisms. If we are dealing with or- 

 ganisms which move about very little, and whose yoimg or 

 eggs or seeds are not commonly transported over great distan- 

 ces, we must recognize that the chance for a varietal difference, 

 depending upon a genotypic difference in one or two genes, to 

 persist in the absence of any selection is materially heightened. 

 In self-tertihzing organisms varieties are incipient species, each 

 variety is fully protected from the random crossing with the 

 great majority of individuals of the tj^e of the species. And 

 now we must admit that in aUogamous organisms, this same 

 chance for a slight genotypic difference to persist within a spe- 

 cies, varies from practically zero in rapidly spreading organ- 

 isms to something which approaches more or less to the same 

 chance in self-fertilized organisms. It is evident that in both in- 

 stances the automatic reduction of the potential variabUity of 

 any group works against the persistence of varietal characters, 

 in a measure proportionate to the percentage of individuals 

 procreating out of total number produced. 



In animals or plants which hardly move about, the chance 

 for inbreeding, and therefore the chance for a mating between 

 individuals with the same genot37pical pecuharity becomes 

 greater, audit is certainly not infinitely small in extreme 

 instances. Snails must for this reason be very much more liable 

 to produce local species than falcons, even if a group of snails 

 covers the same territory as a group of falcons and in ihe absence 

 of barriers, there will be a chance for the snails to produce local 

 species out of its varieties, but none for the falcons. The clear- 

 est instances of circumscribed groups of organisms within 

 which free-crossing is the rule, are the cases in which such 

 groups are locked up into a circumscribed area in the way in 

 which aquatic organisms are confined in a lake. Here we may 



