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Show 358 CLASSIFICATION. [Cn.A.P. XIII. CHAPTER XIII. MUTUAL AFFINITIES OF ORGANIC BEINGS: MORPHOLOGY: EMBRYOLOGY: RUDIMENTARY ORGANS. CLASSIFI<JATION1 groups subordinate to groups- Natural system-Rules and difficulties in classification, explained on the theory of descent with modification- Classification of varieties-Descent always used in classification-Analogical or adaptive characters-Affinities, general, complex and radiating-Extinction separates and defines groups-MORPHOLOGY, between members of the same class, between parts of the same individual- EMBRYOLOGY, laws of, explained by variations not supervening at an early age, and being inherited at a corresponding age-RuDIMENTAnY ORGANS; their origin explained-Summary. FRoM the first dawn of life, all organic beings are found to resemble each other in descending degrees, so that they can be classed in groups under groups. This classification is evidently not arbitrary like the grouping of the stars in constellations. The existence of groups would have been of simple signification if one group had been exclusively fitted to inhabit the land, and another the water; one to feed on flesh, another on vegetable matter, and so on; but the case is widely different in nature; for it is notorious how comn1only members of even the same sub-group have different habits. In our second and fourth chapters, on Variation and on Natural Selection, I have attempted to show that it is the widely ranging, the much diffused and common, that is the dominant species belonging to the larger genera, which vary most. The varieties, or incipient species, thus produced ultimately become converted, as I believe, into new and distinct species ; and these, on the principle of inheritance, tend to produce other new and dominant species. Consequently the groups which are now large, and which generally include many dominant species, tend to go on increasing indefinitely in size. I further attempted to show that from the varying de- CRAP. XIII.] CLASSIFICATION. d 359 seen ants of each speci t . as di~erent places as po:~bf!~~g tt occupy as many and there IS a constant tendency· th . e economy of nature T.h I·S ~one1 u s1· on was supporItne d bm r chara . c t ers to di·v erge.' diversity of the forms of l'!' 1 .Y1 l?oking at the great come I·n t o t h e closest cmnp ltl ·e t. w uc 1 ' In any small area tain facts in naturalisation e I Ion, and by looking to cer~ I attempted also to sh~w th . dency in the forms which a !l't thm:e Is a constant ten-diverging in character to re Increasing in number and less divergent, the les~ im;~~~;~nt and exter~inate the I requ~st the reader to turn to than~ preced.Ing forms. the action, as formerly expla' d fe ~Iagram Illustrating ples, and he will see that th IJ?-e '.0 t ese several princimodified descendants proce ~~vit7:le result is that the become broken up into rou ee Jng r?m one progenitor the diagram each letter ~n th! ~u bordinate ~o groups. In sent a genus including several PP.ermost hne may repreon this line form together on sreme; ; and all the genera ed from one ancient but unsee c ass, or all have descendhave inherited something i:n parent, and, consequently, ~enera on the left hand have co~mon. But the three m c~mmon, and form a sub-f:~iihisd~ai~e principle, much eluding the next two y, Ishnct from that in-diverged from a commo~e;:~:n~~;~~ r~fhtht hand, which scent. These five genera h e stage of de~ ommon ; and they form a fa~:i1 als.? J?Uch, though less, in ~g the three genera still furth y ~Isthnct .from that includdiverged .at a still earlier . er o t e right hand, which descended from (A) for period. ~~d all these genera descended from (I) ' S mttn order distinct from the gener~ descended from a sin 1~ at w_e here have .many species and the genera are ~ncfrogen~tor grouped into genera. families, and orders all u:~~~ ~n, or subordinate to, sub~ grand fact in natur~l h' t e Into one class. Thus the under group, which fr Is o~f of th~ .su?ordination of g;oup sufficiently strike u; iso: I s ~miharity, does not always Naturalists try to my JUdgment fully explained ~es in each ~lass, o:~h~eisf:n:!J.e~~es,_genera, a~d But what 18 meant by th. e Natural Sys. IS system? Some authors |