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|02 - Variations Under Nature||02-04 - Wide-ranging, much diffused, and common Species vary most||10||
Guided by theoretical consideration, I thought that some interesting results might be obtained in regard to the nature and relations of the species which vary most, by tabulating all the varieties in several well-worked floras.
At first this seemed a simple task; but Mr. H. C. Watson, to whom I am much indebted for valuable advice and assistance on this subject, soon convinced me that there were many difficulties, as did subsequently Dr. Hooker, even in stronger terms.
I shall reserve for a future work the discussion of these difficulties, and the tables of the proportional numbers of the varying species.
Dr. Hooker permits me to add that after having carefully read my manuscript, and examined the tables, he thinks that the following statements are fairly well established.
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 questions, hereafter to be discussed.
|02 - Variations Under Nature||02-04 - Wide-ranging, much diffused, and common Species vary most||20||
Alphonse de Candolle and others have shown that plants which have very wide ranges generally present varieties; and this might have been expected, as they are exposed to diverse physical conditions, and as they come into competition (which, as we shall hereafter see, is an equally or more important circumstance) with different sets of organic beings.
But my tables further show that, in any limited country, the species which are the most common, that is abound most in individuals, and the species which are most widely diffused within their own country (and this is a different consideration from wide range, and to a certain extent from commonness), oftenest give rise to varieties sufficiently well marked to have been recorded in botanical works.
Hence it is the most flourishing, or, as they may be called, the dominant species,- those which range widely, 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, perhaps, might have been anticipated; for as varieties, in order to become in any degree permanent, necessarily have to struggle with the other inhabitants of the country, the species which are already dominant will be the most likely to yield offspring, which, though in some slight degree modified, still inherit those advantages that enabled their parents to become dominant over their compatriots.
In these remarks on predominance, it should be understood that reference is made only to the forms which come into competition with each other, and more especially to the members of the same genus or class having nearly similar habits of life.
|02 - Variations Under Nature||02-04 - Wide-ranging, much diffused, and common Species vary most||30||
With respect to the number of individuals or commonness of species, the comparison of course relates only to the members of the same group.
One of the higher plants may be said to be dominant if it be more numerous in individuals and more widely diffused than the other plants of the same country, which live under nearly the same conditions.
A plant of this kind is not the less dominant because some conferva inhabiting the water or some parasitic fungus is infinitely more numerous in individuals and more widely diffused.
But if the conferva or parasitic fungus exceeds its allies in the above respects, it will then be dominant within its own class.
|02 - Variations Under Nature||02-05 - Species of the Larger Genera in each Country vary more frequently than the Species of the Smaller Genera||10||
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 somewhat 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 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 might expect that the circumstances would generally be still favourable to variation.