Seite - 71 - in The Origin of Species

SPECIES OF LARGER GENERA VARIABLE 71 SPECIES OF THE LARGER GENERA IN EACH COUNTRY VARY MORE FREQUENTLY THAN THE SPECIES OF THE SMALLER GENERA If the plants inhabiting a country, as described in any Flora, be divided into two equal masses, all those in the larger genera (i.e., those including many species) being placed on one side, and all those in the smaller genera on the other side, the former will be found to include a some- what larger number of the very common and much diffused or dominant species. This might have been anticipated; for the mere fact of many species of the same genus inhabiting any country, shows that there is something in the organic or inorganic conditions of that country favourable to the genus; and, consequently, we might have expected to have found in the larger genera, or those including many species, a larger proportional number of dominant species. But so many causes tend to obscure this result, that I am surprised that my tables show even a small majority on the side of the larger genera. I will here allude to only two causes of obscurity. Fresh-water and salt-loving plants generally have very wide ranges and are much diffused, but this seems to be connected with the nature of the stations inhabited by them, and has little or no relation to the size of the genera to which the species belong. Again, plants low in the scale of organisation are generally much more widely diffused than plants higher in the scale ; and here again there is no close relation to the size of the genera. The cause of lowly- organised plants ranging widely will be discussed in our chapter on Geographical Distribution. From looking at species as only strongly-marked and well- defined varieties, I was led to anticipate that the species of the larger genera in each country would oftener present varieties, than the species of the smaller genera ; for wher- ever many closely related species (i.e., species of the same genus) have been formed, many varieties or incipient spe- cies ought, as a general rule, to be now forming. Where m.any large trees grow, we expect to find saplings. Where many species of a genus have been formed through varia- tion, circumstances have been favourable for variation; and