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|02 - Variations Under Nature||02-05 - Species of the Larger Genera in each Country vary more frequently than the Species of the Smaller Genera||20||
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 the plants of twelve countries, and the coleopterous insects 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 presented varieties, than on the side of the smaller genera.
Moreover, the species of the large genera which present 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 all the least genera, with from only one to four species, are altogether excluded from the tables.
These facts are of plain signification on the view that species are only strongly-marked and permanent varieties; for wherever many species of the same genus have been formed, or where, if we may use the expression, the manufactory of species has been active, we ought generally to find the manufactory still in action, more especially as we have every reason to believe the process of manufacturing new species to be a slow one.
And this certainly holds true, if varieties be looked at as incipient species; for my tables clearly show as a general rule that, wherever many species of a genus have been formed, the species of that genus present a number of varieties, that is of incipient species, beyond the average.
It is not that all large genera are now varying much, and are thus increasing in the number of their species, or that no small genera are now varying and increasing; for if this had been so, it would have been fatal to my theory; inasmuch as geology plainly tells us that small genera have in the lapse of time often increased greatly in size; and that large genera have often come to their maxima, declined, and disappeared.
All that we want to show is, that when many species of a genus have been formed, on an average many are still forming; and this certainly holds good.
|02 - Variations Under Nature||02-06 - Many of the Species included within the Larger Genera resemble Varieties in being very closely, but unequally, related to each other, and in having restricted ranges||10||
There are other relations between the species of large genera and their recorded varieties which deserve notice.
We have seen that there is no infallible criterion by which to distinguish species and well-marked varieties; and when intermediate links have not been found between doubtful forms, naturalists are compelled to come to a determination by the amount of difference between them, judging by analogy whether or not the amount suffices to raise one or both to the rank of species.
Hence the amount of difference is one very important criterion in settling whether two forms should be ranked as species or varieties.
Now Fries has remarked in regard to plants, and Westwood in regard to insects, that in large genera the amount of difference between the species is often exceedingly small.
I have endeavoured to test this numerically by averages, and, as far as my imperfect results go, they confirm the view.
I have also consulted some sagacious and experienced observers, and, after deliberation, they concur in this view.
In this respect, therefore, the species of the larger genera resemble varieties, more than do the species of the smaller genera.
Or the case may be put in another way, and it maybe said, that in the larger genera, in which a number of varieties or incipient species greater than the average are now manufacturing, many of the species already manufactured still to a certain extent resemble varieties, for they differ from each other by less than the usual amount of difference.
Moreover, the species of the larger genera are related to each other, in the same manner as the varieties of any one species are related to each other. No naturalist pretends that all the species of a genus are equally distinct from each other; they may generally be divided into sub-genera, or sections, or lesser groups.
As Fries has well remarked, little groups of species are generally clustered like satellites around other species.
|02 - Variations Under Nature||02-06 - Many of the Species included within the Larger Genera resemble Varieties in being very closely, but unequally, related to each other, and in having restricted ranges||20||
And what are varieties but groups of forms, unequally related to each other, and clustered round certain forms- that is, round their parent-species. Undoubtedly there is one most important point of difference between varieties and species; namely, that the amount of difference between varieties, when compared with each other or with their parent-species, is much less than that between the species of the same genus.
But when we come to discuss the principle, as I call it, of Divergence of Character, we shall see how this may be explained, and how the lesser differences between varieties tend to increase into the greater differences between species.
There is one other point which is worth notice.
Varieties generally have much restricted ranges: this statement is indeed scarcely more than a truism, for, if a variety were found to have a wider range than that of its supposed parent-species, their denominations would be reversed.
But there is reason to believe that the species which are very closely allied to other species, and in so far resemble varieties, often have much restricted ranges.
For instance, Mr. H. C. Watson has marked for me in the well-sifted London Catalogue of Plants (4th edition) 63 plants which are therein ranked as species, but which he considers as so closely allied to other species as to be of doubtful value: these 63 reputed species range on an average over 6.9 of the provinces into which Mr. Watson has divided Great Britain. Now, in this same Catalogue, 53 acknowledged varieties are recorded, and these range over 7.7 provinces; whereas, the species to which these varieties belong range over 14.3 provinces.
So that the acknowledged varieties have nearly the same
restricted average range, as have the closely allied forms, marked for me by Mr. Watson as doubtful species, but which are almost universally ranked by British botanists as good and true species.
|02 - Variations Under Nature||02-07 - Summary||10||
Finally, varieties cannot be distinguished from species,- except, first, by the discovery of intermediate linking forms; and, secondly, by a certain indefinite amount of difference between them; for two forms, if differing very little, are generally ranked as varieties, notwithstanding that they cannot be closely connected; but the amount of difference considered necessary to give to any two forms the rank of species cannot be defined.
In genera having more than the average number of species in any country, the species of these genera have more than the average number of varieties.
In large genera the species are apt to be closely, but unequally, allied together, forming little clusters round other species. Species very closely allied to other species apparently have restricted ranges.