SHORTER ARTICLES AND DISCUSSION 



HYBRIDIZATION AND EVOLUTION 



Some years ago the writer made a cross between the two species 

 Nicotiana ruslica L. and Nicotiun/t panlculata L. 1 Since the 

 hybrids obtained through this mating are not completely sterile, 

 some biologists may perhaps maintain they are not distinct spe- 

 cies, but such a claim is wholly arbitrary. In a sense, a species 

 is a human concept and as such its definition may be carried to 

 any ridiculous extreme, yet there is no move striking biological 

 fact than that in general the great groups of living things do fall 

 into specific subdivisions which many criteria show to be distinct, 

 discontinuous, without intermediates. In two such groups fall 

 the above types. Though their ranges overlap, they differ from 

 each other in leaf, stem, flower and habit of growth much more 

 than do several other pairs of species within the same genus be- 

 tween which hybridization is impossible, or where the hybrid is 

 sterile. 



The cross between these two species gives an F t generation in- 

 termediate between the two parents, and as uniform in each char- 

 acter as either parental group. 



Few of the male or the female gametes are viable, yet by care- 

 ful attention to pollination, front one to twenty seeds can be 

 obtained in the capsules, where normally two hundred to three 

 hundred seeds are found. These seeds produce an P 2 generation 

 which is inordinately variable. No two plants are similar, and 

 numerous types can be picked out which if found in the wild 

 would undoubtedly be classed as different species. In genetic 

 terms, the behavior of the two species may be described as fol- 

 lows: They differ in an extremely large number of inherited 

 factors; and owing to these numerous differences, many of the 

 otherwise possible combinations of F 1 gametes, are not func- 

 tional. A huge percentage of expected combinations of both 



g The factors^ which in combination produce normal fertility, 

 Joe., 54: 7(M% 1915. 



262 



