1 88 SELECTION. 



In the second place, only isolation of some kind can produce 

 species, after the first condition has been fulfilled. For selec- 

 tion alone or fitness alone — cannot make a species out of the 

 most happily constituted variety without isolation. In domestic 

 plants and animals we saw that selection meant isolation, 

 and it must be remembered that in nature, selection, that is 

 survival because of merit, may become a means of isolation, 

 and so a factor in species formation. In some instances special 

 ability to survive in conditions into which the bulk of the in- 

 dividuals of the species cannot penetrate, can produce an iso- 

 lation in space. This sort of isolation is obviously much more 

 effective than chance isolation in space without special con- 

 stitutional adaptation for this new habitat, for in the second 

 instance swamping by the multitude of the old species is only 

 a matter of time and chance This, to our way of thinking, is, 

 where natural selection tends to influence the constitution of 

 the surviving species. Only a constitutional difference from 

 parent-forms sufficient to produce a preponderance of matings 

 between members of the group over matings with individuals 

 of the old group can isolate a new species sufficiently to make 

 it survive in close proximity of the old one. 



Autogamy is the most common mode of complete isolation 

 thinkable., Where self-fertilization is the rule, and the same is 

 true for any other process of multiphcation without amphi- 

 mixis, such as parthenogenesis and apogamy, new groups of 

 organisms which to all practical purposes are species, will be 

 found to aboimd. Habitual self-fertihzation, or habitual apog- 

 cimy will tend to restrict the crossing, and wiU therefore 

 counteract a heightening of the potential variability. But in 

 these groups, every individual, no matter what his genotypic 

 constitution, is a potential species, of the same rank as a 

 group of allogamous organisms which has under the influence 

 of some kind of isolation reduced its potential variability and 

 has attained a type of its own. 



Selection in the case of these autogamous organisms means 

 survival or dying out of these small species. Those which are 



