ECOLOGICAL BASIS 13 



out by the nonsegregating habitat in which they would have 

 to hve. As a consequence, it is only where man or cata- 

 strophic natural forces have ''hybridized the habitat" that 

 any appreciable number of segregates survives. It will be 

 well to expand this condensation and outline the critical evi- 

 dence on which it is based. 



The key to understanding the reaction between hybrid 

 segregates and the environment is the realization that hab- 

 itat preferences are inherited in substantially the same 

 fashion as any other character. We now know that physi- 

 ological differences are inherited in the same w^ay as mor- 

 phological ones; some of them are single-factor differences, 

 whereas many of them are multifactorial. The lower or- 

 ganisms are more practical subjects for laboratory research, 

 and it is in such fungi as Neurospora (Beadle, 1945) and 

 yeast (Lindegren and Lindegren, 1947) that the inheritance 

 of physiological differences has been worked out in greatest 

 detail. Similar studies have been made in the higher plants, 

 and for a few characters, such as reaction to length of day 

 and the genetic control of the auxin mechanisms, fairly pre- 

 cise results have been obtained. 



In any cross between two species, therefore, the inherent 

 differences that allow them to fit into different habitats 

 segregate in the same manner as morphological ones. The 

 Fi is as uniform as the parental species; the F2 is highly 

 variable. The preferences of • first-generation hybrids are 

 substantially alike and are more or less intermediate be- 

 tween those of the two parents. In succeeding hybrid genera- 

 tions or backcrosses these inherent differences recombine 

 variously. Just as most of the hybrids of the second genera- 

 tion represent different recombinations of the morpho- 

 logical characters of the parents so that no two look exactly 

 alike, so the habitat preferences of these same plants vary 

 from individual to indi\ddual. Though they came from 

 species that were each essentially uniform in their require- 

 ments for an optimum habitat, this second generation is 

 made up of indi\dduals each of which differs from the rest 



