OCR Text |
Show 56 SPECIES OF LARGE GENERA CHAP. 11. facts are of plain signification on the view t~at. species are only strongly marked and permanent varwtles ; for wherever many species of the same genus ha:e 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 ~ction, more especially as we have every reason to beheve the process ~f manufacturing new species to be a slow one. And this certainly is the case, 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 where many species of a genus have been formed, on an average many are still forming; and this holds good. 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 in those cases in which 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 CHAP. II. RESEMBLE VARIETIES. 57 should be ranked as species or varieties. Now Fries has remarked in regard to plants, and Westwood in r~gard 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 always confirm the view. I have also consulted some sao-acious and most experienced observers, and, after delib:ration, they. concur in this view. In this respect, therefore, the spemes of the larger genera resemble varieties, more than do the species of the smaller genera. Or the case :nay be put in another way, and it may be said, that In the larger genera, in which a number of varieties or incipient species greater than the average are now manufac.turing, many of the species already manufactured still to a certain extent resemble varieties for they differ from each other by a less than usual am~unt of difference. Moreover, the species of the large. genera are related to each other, in the same manner as the varieties of any one species are related to each other. No natu-· r~li~t pretends that all the species of a genus are equally ~1st1nct from each other; they may generally be divided 1nto sub-genera., or sections, or lesser groups. As Fries has well r~marked, .little groups of species are generally clustered hke satellites around certain other species. And what are varieties but groups of forms, unequally related to each other, and clustered round certain forms-that is roun~ their parent-species? Undoubtedly there is on~ mos~ Important point of difference between varieties and spe?Ie.s; namely, that the amount of difference between vaneties, when compared with each other or with their p~rent-species, is much less than that between the speCies o~ t~e same genus. But when we come to discuss the pnnmple, as I call it, of Divergence of Character, D3 |