GENERAL DISCUSSION. 97 



G. THE ROLE OF HYBRIDIZATION IN EVOLUTION. 



The criticism has often been made of modern studies in hybridization 

 that they are really unimportant for evolution because hybridization is 

 uncommon in nature. Even at the beginning of the new era it could be 

 replied that, first, we did not know how common hybridization might 

 turn out to be in nature, and, second, that certainly in human marriage 

 and among domesticated animals and plants, intermixing of characters 

 played a most important part, and, finally, the laws of inheritance of char- 

 acters were of such grave physiological import as to deserve study whoUy 

 apart from any question of the r61e of hybridization in evolution. 



The last decade of work has made clear many things that were before 

 uncertain. We now realize that in nature hybridization may and actually 

 does proceed extensively. Dr. Ezra Brainerd has shown how many wild 

 "species" of Viola have arisen by hybridization, as may be proved by 

 extracting from them combinations of characters that are found in the 

 species that are undoubtedly ancestral to them. In such highly variable 

 animals as Helix nemaralis and Helix hortensis it is veiy probable that 

 individuals with dissimilar characters regularly mate in nature and trans- 

 mit diverse combinations of characters to their progeny. Indeed, if one 

 examines a table of species of a genus or of varieties of a species one is 

 struck by the paucity of distinctive characters. The way in which species, 

 as found in nature, are made up of different combinations of the same 

 characters is illustrated by the following example, taken almost at random. 

 Among the earwigs is the genus Opisthocosmia, of which the 5 species known 

 from Sumatra alone may be considered. They differ, among other qualities, 

 chiefly in the following characters (Bormans and Kraus, 1900): 



Size: A, large; a, small. 



Wing-scale: B, brown; b, yellow. 



Antennal joints: C, unlike in color; c, uniform. 



Forceps at base: D, separated; d, not separated. 



Edge of forceps: E, toothed; e, not toothed. 



Fourth and fifth abdominal segments: F, granular; /, not granular. 



The combinations of these characters that are found are as follows: 



Opisthocosmia omata: Abe DBF. 

 insignis: ABc DE f. 

 longipes: A b CDEf. 

 tenella: AbCde f. 

 minuscula: aBCDEf. 



Other species occur, in other countries, showing a different combina- 

 tion of characters, and there are characters not contained in this list, 

 which is purposely reduced to a simple form; but the same principles apply 

 generally. 



The bearing upon evolution of the fact that species are varying combi- 

 nations of relatively few characters is most important. Combined with the 



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