438 EVOLUTION, GENETICS, AND EUGENICS 



/) Psychic isolation. — A synonym for psychic isolation is "clan- 

 nishness." Even among lower animals there appears to be a well- 

 defined tendency for like to mate with like. "Assortative mating" 

 is another term commonly used to denote this tendency. In man 

 conventional mating is highly assortative. Sporadic sex unions be- 

 tween members of radically different races not uncommonly occur, 

 but formal marriages are rare between members of distinct races. 

 This tends to preserve a residue of racial separateness that would long 

 ago have disappeared except for clannishness. 



Examples of this sort of thing in the animal world are not far to 

 seek. Beebe cites the interesting case of two color phases of gannets 

 occupying an old volcanic crater together, but keeping absolutely 

 separate in their breeding activities. The main gannet population 

 consisted of white birds; but out toward the center of the crater there 

 was a small, compact, and unmixed clump of sooty birds, which could 

 doubtless have readily interbred with whites, but never did so, prefer- 

 ring their own kind. 



It was Darwin, I believe, who described the case of droves of wild 

 horses of the Caucasus region that were all of one color in a drove. 

 One drove would be all bays; another all blacks; another all chest- 

 nuts. Apparently these droves consisted of closely related individuals 

 that preferred to mate only with their own kind. Long-continued 

 reproductive isolation of this sort might readily bring about the origin 

 of separate races, and in time separate species. 



Most of the types of isolation briefly discussed above are somewhat 

 hypothetical in that they are derived from observations of nature 

 "after the event," as it were. Very little real experimental work has 

 been done in this field except in connection with inbreeding and se- 

 lection. In animal and plant breeding, man merely isolates a pair of 

 individuals, usually beginning with a brother and sister, and begins 

 an inbred line, selecting usually the most promising individuals for 

 further inbreeding. In this way excellent homozygous strains are 

 produced that have peculiar characteristics of their own unlike those 

 of any other strain. Hence, artificial inbreeding and selection is a 

 kind of reproductive isolation having the same effects as has extreme 

 isolation in nature. There can be no doubt, then, that the various 

 types of isolation have the effects attributed to them, namely, those of 

 splitting up species and of preserving incipient races from being 

 swamped out by back-crossing with the parent stock. 



