ECOLOGICAL BASIS 15 



zygotes were not formed, but because there was no ''re- 

 combination habitat" in which they could sur\dve. It is 

 usually only through the intervention of man that such 

 multiple habitats are even approximated. When he digs 

 ditches, lumbers woodlands, builds roads, creates pastures, 

 etc., man unconsciously brings about new combinations of 

 light and moisture and soil conditions. At such time he may 

 be said to ''hybridize the habitat," and it is significant that 

 many of the careful studies of hybridization in the field have 

 been made in such areas. As to the way in which the same 

 effects can under certain circumstances occur without the 

 intervention of man, see Chapter 5, pp. 62 to 66. 



Even where man has "hybridized the habitat," most of the 

 new recombination habitats are fairly close to one of the 

 original ones. In such areas, therefore, we may generally 

 expect to find recombination plants closely resembling one 

 of the parental species. The hybrids and backcrosses most 

 likely to survive will be those very similar to one or the 

 other of the parents. The restrictive effect of the environ- 

 ment will be to limit the results of hybridization in nature 

 very largely to backcrosses. Among them, the environment 

 will ordinarily give greatest preference to those backcrosses 

 most like the recurrent species. 



The greater the number of gene differences between the 

 parents, the greater will be the number of special new hab- 

 itats necessary for the segregates. Everything else being 

 equal, we shall expect the lack of recombined habitats to be 

 the stronger barrier, the greater the differentiation between 

 two hybridizing entities. 



If 2 hybridizing entities are differentiated by only 1 pair 

 of genes affecting habitat preferences, the F2 will demand 

 only these 2 habitats and their intermediate condition. If 

 there are 2 pairs of differentiating genes, we need 4 habitats ; 

 if there are 3 differences, we require 8. With only 10 such 

 differences 1024 habitats are required; and with 20, over 

 1,000,000. Let us see exactly what these figures mean. As- 

 suming no other barriers and no inherent disharmonies in the 



