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Show 54 DOMIN ANT SPECIES VARY MOST. [CHAP. II. I-Iooker even in stroncrer terms. I shall reserve for 1ny future ~vork the disc~ssion of these difficulties, and the tables thmnselves of the proportional nun1bers of the varying species. Dr. I-Iooker permits .me to add, t~at after having carefully read my manuscnpt, and examined .the tables he thinks that the following statmnents are fauly well c~tablished. The whole subject, however, treated as it necessarily here is with much brevity, is rather perplexing, and allusions cannot be avoided to the "struggle for existence" " divergence of character," and other ques- tions, hereaft' er to be di.s cussed. Alph. De Candolle and others have shown that pl~nts which have very wide ranges generally present vanetles; and this 1nicrht have been expected, as they become exposed to d~erse physical conditions, and as theJ: come into co1npetition (~hich, as .. we sh~ll h~r~after see, IS a far more important cn·cun1stance) With different sets . of organic beinO'S. But 1ny tables further show that, In any limited cou~try, the species which are most ~ommo:n, that is abound most in individuals, and the speCies ·wh1ch ai:e rdost widely diffu~ed w~thin their .~wn country (and tlns is a different consideration from Wide range, and to a certain extent from commonness), often give rise t~ varieti~s sufficiently well-ma~k~d to have been r.ec~rded In botanical works. Hence 1t IS the most flourishing, or, as they n1ay be called, the dominant species,-. those ~vhich. range widely over the world, are the most diff':se~ In. t~1e1r own country, and are the most numerous _111. Individuals,which oftenest produce well-marked va~·Ietles, or, as I ?onsider them, incipient species. An~ ~his, .perhaps, m1ght have been anticipated ; for, as varieties~ In order to become in any degree permanent, necessarily have to str~ggle with the other inhab.itants o~ the country, the. speCies which are already dominant will be the Inost hkely t? yield offspring which, though in some slight degree mod_Ified will still inherit those ad vantages that enabled theu par~nts to become doininant over their con'lpatriot~. . If the plants inhabiting a country and descnbed :n any Flora be divided into two cq nal masses, all those 111 the larger genera being placed on one side, aud all those CHAP II.] SPECIES OF LARGE GENERA VARIABLE. 55 in the smaller genera on the other side, a somewhat larger number of the very common and n1uch diffused or domina: r;t spec~es will be found on the s~d.e 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 1nany 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 muci1 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 ag,ain there is no close relation to the size of the genera. The cause of lowly-organised plants ranging widely will be discussed in ovr 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 wherever many closely related species (i.e. species of the same genus) have been formed, many varieties or incipient species ought, as a general rule, to be now forming. Where 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 1night expect 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. |