GENETIC BASIS 23 



eration hybrids, sterile m^raspecific crosses, etc. Modern 

 cytology has shown the special features that produce these 

 exceptions and now includes all these seeming exceptions 

 under one general theory. We shall restrict the following 

 discussion to the commonest and most general kinds of hy- 

 brids, those which (in Darlington's terminology) come from 

 unhke parents and give rise to unlike offspring. The general 

 results of such hybridizations have again been known since 

 the times of Koelretuer and Von Gartner (Plates 4 and 5). 

 The first hybrid (Fi) generation is uniform, sometimes 

 strikingly so. Aside from differences due to the extreme 

 vigor that tends to characterize such hybrids, it is morpho- 

 logically intermediate between the two parents. On the 

 other hand, the second generation (F2) characteristically 

 varies (Plate 4) from individual to individual. If raised by 

 the tens or by the hundreds, seldom are there two individuals 

 with exactly the same combination of parental character- 

 istics. In general, a large F2 can be sho\\TL to pass from a few 

 recombinations very similar to one of the parents, to a great 

 variety of intermediates — the majority of which are fairly 

 similar to the Fi — to a relatively few individuals very much 

 like the other parent. 



If the Fi is backcrossed to the two parental species, each 

 of these backcross generations varies from indi\ddual to in- 

 dividual, though not so markedly as the F2. In such back- 

 crosses (Plate 5) usually a few individuals are almost in- 

 distinguishable from the recurrent parent (i.e., the one to 

 which they have been backcrossed), and a large number are 

 in various ways intermediate between this parent and the 

 Fi. A few will be rather similar to the Fi itself. If any of 

 these first backcrosses are again crossed back to the same 

 parent the resulting progeny vary even less among them- 

 selves and are in general very similar to the recurrent parent. 

 After a succession of 5 or 6 such backcrosses they usually 

 become indistinguishable from the recurrent parent. 



Genetics has given us a sound theoretical basis for inter- 

 preting these results. The multiple-factor hypothesis ex- 



