14 INTROGRESSIVE HYBRIDIZATION 



in its requirements. The same is true of the backcrosses. 

 Just as they are characterized morphologically by individuals 

 that vary somewhat among themselves but as a whole are 

 fairly similar to their recurrent * parental species, so they 

 are characterized physiologically by individuals whose require- 

 ments are somewhat variable though as a whole are fairly 

 close to those of the parent to which they were backcrossed. 



In nature, therefore, the problem of survival is very dif- 

 ferent for the first and for succeeding hybrid generations. 

 If two species inhabiting two different habitats are crossed 

 under natural conditions, the first hybrid generation can be 

 expected to survive if there are occasional intermediate 

 zones in which conditions as a whole are somewhat inter- 

 mediate between that of the two habitats. All the individ- 

 uals of the first hybrid generation are substantially alike, 

 differing no more among themselves than did the individuals 

 of the more variable parental species. Furthermore, as a 

 result of their hybridity, they ordinarily are vigorous and, 

 once established, may (depending on the degree of their 

 hybrid vigor) be more capable of maintaining themselves 

 than an ordinary nonhybrid. The progeny of these first- 

 generation hybrids, however, presents quite a different prob- 

 lem. Each of them prefers a slightly different habitat. Their 

 preferences as a whole run from something more or less like 

 that of one species, through a whole series of varying inter- 

 mediate conditions, to something more or less hke that of the 

 other parent. 



Multiple habitats such as would be demanded if any con- 

 siderable portion of the segregating hybrid generations were 

 to survive are seldom met with in nature. Even if complex 

 hybrid swarms are growing under natural conditions, a 

 repetition of the cross in an experimental garden reveals 

 whole groups of hybrids and backcrosses that were not found 

 in the wild population. They were missing not because such 



* FolIo\\'ing general usage by plant breeders, we shall refer to the 

 parental species to which the hybrid has been backcrossed for one or 

 more generations as the recurrent parental species. 



