THE PRINCIPLES OF CLASSIFICATION. 321 



635. One distinction between varieties and species is note- 

 worthy and important, even if it may not serve as a criterion. 

 The individuals of different varieties in plants interbreed as freely 

 as do those of the same variety and are equally prolific. Their 

 union produces 



636. Cross-breeds. 1 In nature, cross-breeding doubtless re- 

 presses variation or prevents the segregation of varieties into 

 what would be ranked as species. In cultivation and domesti- 

 cation, it is turned to important account in producing intermediate 

 new varieties (cross-breeds) variously combining the different 

 excellencies of two parent individuals or two varieties. Thus 

 the great number of forms produced by variation (especially as 

 to flowers and fruits) have been further diversified, and selected 

 forms improved for special uses by judicious combination. 



637. In general, the individuals of distinct species do not 

 interbreed, although many are capable of it. There is great 

 diversity in this regard among plants, some (such as Willows, 

 Verbascums, and Verbenas) interbreeding freely and reciprocally ; 

 some interbreeding in one direction, but not reciprocally ; others, 

 even when very similar, refusing to unite. But, on the whole, 

 there seems to be few nearly related species in which the pollen 

 of the one cannot be made to act upon the ovules of the other 

 by persistent and proper management. Such crossing is an 

 important resource in horticulture. Crossing of species, when 

 successful, produces 



638. Hybrids. In these, the characteristics of the two species 

 are combined, sometimes in equal proportions, sometimes with 

 great preponderance of one or the other parent ; and there is 

 often a difference in the result in reciprocal fertilizations. Hy- 

 brids do not play a very prominent part in nature, apart from 

 cultivation, although the limits of some species may be obscured 

 by them, possibly of more than is generally supposed. In the 

 animal kingdom, all the most familiar hybrids are sterile : in the 

 vegetable kingdom, a majority may have a certain but very low 

 degree of fertility ; but this is also the case in many unions 



May, 1869, 5) as the common designation of any group or collection of indi- 

 viduals whose characters are continued through successive generations, 

 whether it be permanent variety, subspecies, species, or group consisting 

 of very similar species, the term not implying any decision of this question. 

 If this use of the term race prevails, Subspecies will probably take its place 

 as the designation of the highest grade of variety. The objection to this is 

 that the subspecific and specific names would be more liable to be confused. 

 1 Half-breed is a common equivalent term in the animal kingdom : Latin, 

 Mistua or Mixtus ; French, Metis. 



II 



