| OCR Text |
Show 46 V A.RIA.TION UNDER NATURE. CHAP. II. by slow degrees: yet quite recently Mr. ~ubbock h~s shown a degree of variability in these mam n_erves In Coccus, which may almost be compared ~o th~ rrreg~ar branching of the stem of a tree. . This philosophical naturalist, I may add, has also quite r~ce~tly shown that the muscles in the larvre of certain msects are very far from uniform. Auth~rs sometimes argue in a circle when they state that Important organs never vary; for these same authors practicall! rank that character as important (as some few naturalists have ~ones~ly confessed) which does not ~ary; and, under t~s pm~t of view, no instance of an Important part varying will ever be found : but under any other point of view many instances assuredly can be given. There is one point connected with individual differ-ences, which seems to me extremely perplexing: I refer to those genera which have sometimes been called "protean" or "polymorphic," in which the species present an inordinate amount of variation ; and hardly two naturalists can agree which forms to rank as species and which as varieties. We may instance Rubus, Rosa, and Hieracium amongst plants, several genera of insects, and several genera of Brachiopod shells. In most polymorphic genera some of the species have fixed and definite characters. Genera which are polymorphic in one country seem to be, with some few exceptions, polymorphic in other countries, and likewise, judging from Brachiopod shells, at former periods of time. These facts seem to be very perplexing, for they seem to show that this kind of variability is independent of the conditions of life. I am inclined to suspect that we see in these polymorphic genera variations in points of structure which are of no service or disservice to the species, and which consequently have not been seized on and rendered definite by natural selection, as hereafter will be explained. CHAP. II. DOUBTFUL SPECIES. 47 Those forms which possess in some considerable ~eg~ee the character of species, but which are so closely Similar to_ some ot~er forms, or are . so closely linked to t~em by Intermediate gradations, that naturalists do not hke to rank them as distinct species, are in several respects the most important for us. We have every reason to believe that many of these doubtful and closely-allied forms have permanently retained their characters in their own country for a long time; for as long, as far as we know, as have good and true species. Practically, when a naturalist can unite two forms together by others having intermediate characters, he treats the one as a v~riety of the other, ranking the most common, but sometrmes the one first described, as the species, and the o~her as t~e variety. But cases of great difficulty, which I will not here enumerate sometimes occur in deciding whether or not to rank 'one form as a variety o~ anothe~, eve_n when they are closely con_ nected by mt~rmediate links; nor will the commonlyassumed hybnd nature of the intermediate links always remove t~e difficulty. In very many cases, however, one ~orm Is ranked as a variety of another, not because the Intermediate links have actually been found, but because analogy leads the observer to suppose either that the! do now somewhere exist, or may formerly have existed; and here a wide door for the entry of doubt and conjecture is opened. Hence, in determining whether a form should be :anked ~s a species or a variety, the opinion of naturalIsts haVIng sound judgment and wide experience seems the only ~uide to follow. We must, however, in many cases, demde by a majority of naturalists, for few wellmarked and well-known varieties can be named which have not been ranked as species by at least some competent judges. |
