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 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 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; _f_, not granular. The combinations of these characters that are found are as follows: _Opisthocosmia ornata_: _AbcDEF_. _insignis_: _ABcDEf_. _longipes_: _AbCDEf_. _tenella_: _AbCdef_. _minuscula_: _aBCDEf_. Other species occur, in other countries, showing a different combination 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 combinations of relatively few characters is most important. Combined with the fact of hybridization it indicates that the main problem of evolution is that of the origin of specific characteristics. A character, once arisen in an individual, may become a part of any species with which that individual can hybridize. Given the successive origin of the characters _A_, _B_, _C_, _D_, _E_, _F_, in various individuals capable of intergenerating with the mass of the species, it is clear that such characters would in time become similarly combined on many individuals; and the similar individuals, taken together, would constitute a new species. The adjustment of the species would be perfected by the elimination of such combinations as were disadvantageous. COLD SPRING HARBOR, NEW YORK, _May 20, 1909_. LITERATURE CITED.