OCR Text |
Show 52 DOUB'l'FUL SPECIES. [CII.AP. II. to one class within one country, he will soon make_up his mind ho\v to rank most of the doubtful forms. I-I1s general tendency wil~ be t.o make I?any species, for he ~ill become hnpressed, JUSt hke the p1geon ~r _poultr~ fancier before alluded to with the amount of difference 1n the forn1s which he is co~tinually studying; and he has little general know ledge of analogica~ variation in ot~er gro"?-ps and in other countries, by wh1ch to corre?t Ius first. Impressions. As he extends the ra~ge of h1s observ~t1ons, he will meet with more cases of difficulty ; for he w1ll encounter a greater n?-mber of closely-allie~ f~rms. But if his observations be w1dely extended, he w1ll 1n the end generally be enabled to make up his own ~nind which .to ca~l varieties and which species; -but he w1ll succeed 1n th1s at the expense of admitting much variation,-and the truth of this ad1nission will often be disputed by other naturalists. When, moreover, he comes to study allied forms brought from countries not now co~tinuous,_ in which case he can hardly hope to find the 1ntermed1ate links between his doubtful forms, he will have to trust almost entirely to analogy, and his difficulties will rise to a climax. Certainly no cle~r line of demar~ation has. as yet been drawn between species and sub-spec1e~-that 1s, the forms which in the opinion of some naturalists come very near to but do not quite arrive at the rank of species; or, again betw·een sub-species and well-tnarked varieties, or betw~en lesser varieties and individual differences. These differences blend into each other in an insensible series; and a series impresses the mind ·with the idea of an actual passage. Hence I look at individual differences, though of small interest to the systematist, as of high_ import~n?e for us, as being the first step towards such shght varieties as a;e barely thought worth reco~di~g in ~orks o~ natural lnstory. And I look at varieties wh1ch are 1n. any degree more distinct and permanent, as steps le~d1~g to 1nore strongly marked and n1ore perm~nent var1et1e~; and at these latter, as leading to sub-spemes, and to species .. The passage fro1n one stage of difference to another and h1gher Cn.AP, II.) VARIETIES GRADUATE INTO SPECIES. 53 s~age may be, in some cases, due merely to the long-continued action of different physical conditions in two different regions; but I have not much faith i.n this view· an~ I a~tri~ute the pas~age of a variety, fr01n a state i~ wh~ch .1t ~1ffers very shghtly f~om its parent to one in whwh It d1ffers more, to the action of natural selection in a?cumulating (as will hereafter be more fully explained) differences of structure in certain definite dhections. l-Ienee I ~eli_ev:e a well:marked variety may be justly called an 1nmp1ent species; but whether this belief be justifiable must be judged of by the general weiO'ht of the several facts and views given throughout this w~rk. It need not be supposed that all varieties or incipient species necessarily attain the rank of species. They may whilst in this i~ci.pient state become extinct, or they may endure as varieties for very long periods, as has been shown to be the case by Mr. Wollaston with the varieties of certa~n fossil land-shells _in Madeira. If a variety were ~o flou~1sh so as to exceed1n n"?-1nbers the parent species, 1t ":ould the~ ra~k as the spemes, and the species as the var1ety ; or 1t 1n1ght come to supplant and exterminate the parent species ; or both might co-exist, and both rank as independent species. But we shall hereafter have to return to this subject. From these remarks it will be seen that I look at the term species, as one arbitrarily given for the sake of convenience to a set of individuals closely resembling each other, and that it does not essentially differ from the term yariety, which is given to less distinct and more fluctuatIng forms. The term variety, again, in comparison with mere individual differences, is also applied arbitrarily, and for 1nere convenience sake. Guided by theoretical considerations, 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 subseg_uently Dr. |