Hybrids 517 



abundant in other localities. Thus different strains or subspecies 

 may segregate out which can be identified from their external 

 appearance but are not so different from one another as to war- 

 rant their being classed as separate species. The individuals that 

 comprise a subspecies may differ somewhat from one another 

 but may resemble one another more than they resemble those 

 that make up another subspecies. The various subspecies also 

 differ from one another, but the individuals of any subspecies 

 resemble those of another subspecies of the same species more 

 closely than they resemble individuals of another species. In- 

 dividuals of one subspecies may frequently cross with those of 

 another if the opportunity presents itself, and in a number of 

 genera we have found that members of one species can cross 

 with those of another. One of the important phases of the species 

 problem is the mechanisms which normally prevent two subspe- 

 cies or two species from crossing but occasionally can break 

 down so as to permit the production of intraspecific or of inter- 

 specific hybrids. 



Hybrids 



A cross between two plants or animals of different types is 

 known as a hybrid, but as the term has been used in several dif- 

 ferent senses, we might well ask whether any inherited difference 

 is sufficient for the application of the term or whether the dif- 

 ferences between the two parents must be of a certain order of 

 magnitude. 



Originally a hybrid was considered to be the offspring of two 

 dift'erent species, genera, or at least races, forms, or subspecies, 

 but with the pubhcation of Mendel's classic paper on genetics a 

 new significance was given the term. Mendel crossed peas which 

 differed by only one, two, or a few genes and yet he applied the 

 term "hybrid" to the offspring of such crosses. This use of the 

 term for crosses between individuals of the same subspecies, form, 

 or race was new. Today such heterozygous individuals are 

 often referred to as mendelian hybrids or gene hybrids. Apply- 

 ing this same principle not to differences in individual genes but 

 to differences in genomes, Darlington and others have used the 

 term hybrid for any zygote which arises as the result of a union 

 of two gametes dissimilar in any respect whatsoever or for a 

 zygote or product of a zygote which produces gametes dissimilar 



