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Show 54 DOMINANT SPECIES VARY MOST. CHAP. II. those which range widely over the world, are the most diffused in their own country, and are the most numerous in individuals,-which oftenest produce well-marked varieties or as I consider them, incipient species. And this, pe;ha~s, might have been anticipated ; for, as varieties in order to become in any degree permanent, necessarlly have to struggle with the other inhabi~ants of the country, the species which are already dominant will be the most likely to yield offspring which, though in some slight degree modified, will still inherit those advantages that enabled their parents to become dominant over their compatriots. If the plants inhabiting a country and described in any Flora be divided into two equal masses, all those in the larger genera being placed on one side, and all those in the smaller genera on the other side, a somewhat larger number of the very common and much diffused or dominant species will be found on the side of the larger genera. This, again, 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 large 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 have generally 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 CrrAP. II. . SPECIES OF LARGE GENERA VARIABLE. 55 ?enerally much more widely diffused than plants higher In the scale; and hete again there is no close relation to the size o.f the ? enera.. The cause of lowly-organised plants ranging widely Will be discussed in our chapter on geographical distribution. 1:!-,rom 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 wherever many closely related species (i.e. species of the same genus) have been formed, many varieties or incip.ient species ought, as a general rule, to be now forming .. Wh~re many large trees grow, we expect to find saplings. Where many species of a genus have been formed through variation, circumstances have been favourable. for variation ; and hence we might ·expec.t that the Circumstances would generally be still favourable to variation. On the other hand, if we look at each species as a special act of creation, there is no apparent reason why more varieties should occur in a group having many species, than in one having few. To test the truth of this anticipation I have arranged ~he plants of .tw~lve countries, and the coleopterous 1nse~ts of two districts, into two nearly equal masses, the species of the larger genera on one side, and those of the smaller genera on the other side, and it has invariably proved to be the case that a larger proportion of the species on the side of the larger genera present varieties than on the side of the smaller genera. Moreover th~ . ' ~pem~s of the large genera which preRent any varieties, Invariably present a larger average number of varieties than do the species of the small genera. Both these results follow when another division is made, and when a~l the smallest genera, with from only one to four speCies, are absolutely excluded from the tables. These |
