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Show 108 CONDITIONS AFFECTING THE The next question, and it is an important one ~or . th" . Is there any limit to the amount of vana-us, IS IS • d tion from the primitive stock which can be pr~du~e bv this process of selective breeding? In considenng h "'. t" "t will be useful to class the character-t 1s ques Ion, 1 · . . . t" . respect of which organic bmngs vary, under lS lCS, Ill . . h d ay Consider structural characteristiCs, two ea s : we m . . and we may consider physiological charactenstics. In the first place, as regards structural charac-t. eri.s t"1 cs, I en deavoured to show you, by the skeletons which I had UlJon the table, and by reference to a great many well-ascertained facts, that the different breeds of Pigeons, the Carriers, Pouters, and Tumble1·s, might vary in any of their internal and important structural characters to a very great degree ; not only might there be changes in the proportions of the skull, and the characters of the feet and beaks, and so on ; but that there might be an absolute difference in the number of the vertebrre of the back, as in the sacral vertebrre of the Pouter; and so great is the extent of the variation in these and similar characters that I pointed out to you, by reference to th~ s~eletons and the diagrams, that these extreme vaneties may absolutely differ more from one another in their .sti:uctural characters than do what naturalists call distinct SPECIES of pigeons; that is to say, that they differ so much in structure that there is a greater difference between the Pouter and the Tumbler than there is between such wild and distinct forms as the Rock Pio-eon or the Ring Pigeon, or the Ring Pigeon and th: Stock Dove ; and indeed the differences are of greater value than this, for the structural differences PERPETUATION OF LIVING BEINGS. 109 between these domesticated pigeons are such as would be admitted by a naturalist, supposing he knew nothing .at all about their origin, to entitle them to constitute .even distinct genera. As I have used this term SPECIES, and shall probably use it a good deal, I had better perhaps devote a word or two to explaining what I mean by it. Animals and plants are divided into groups, which become gradually smaller, beginning with a KINGDOM, which is divided into Sun-l(JNGDO.MS; then come the smaller divisions called PRoVINCEs ; and so on from a PROVINCE to a CLAss, from a CLAss. to an ORDER, from ORDERs to FAMILIEs, and from these to GENERA, until we come at length to the smallest groups of animals which can be defined one from the other by constant ,characters, which are not sexual; and these are what naturalists call SPECIES in practice, whatever they may do in theory. . ~f in a state of nature you find any two groups of hv1ng beings, which are separated one from the other by some constantly -recurring characteristic, I don't care how slight and trivial, so long as it is defined ~nd .. constant, and does not depend on sexual pecuhantles, then all naturalists agree in calling them two species; that is what is meant by the use of the word species-that .i s to say, it is, for the practical naturalist, a mere questiOn of structural differences.* We have seen now-to repeat this point once more and it is very essential that we should rightly under~ * I lay stress here on the practical signification of " Species " Whether a physiological test between species exist or not . t .. hardly ever applicable by the practical naturalist. ' I IS |