No. 543] FACTORS AND METHODS OF EVOLUTION 131 



four forms of leaf in the rosette stage; fruit-flies (Droso- 

 phila) may have any one of several colors of eye com- 

 bined with either short or long wings, and so on. The 

 characteristics that are associated in an individual are, 

 for the most part, not necessarily associated. The group 

 of characteristics that distinguishes individuals of one 

 "species" from those of another is largely an accidental 

 one ; and it is, therefore, not surprising that we so often 

 find individuals which in one, two or several characters 

 differ from the conventional description of their species, 

 and these have in the past caused great difficulty to the 

 species maker. 



The fact that most characteristics are not necessarily 

 associated — that they may occur in various combinations 

 — certainly accounts for the multiplicity of "varieties" 

 in domesticated species ; and for much of the variation in 

 feral species. Moreover, it probably accounts for the 

 presence of many " species" in a genus. I may repeat 

 here what I wrote in 1909. 



Dr. Ezra Brainerd lias 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 nemoralis and Helix hortensis it is very probable that individuals 

 with dissimilar characters regularly mate in nature and transmit 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 

 five 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, granul. 



