OCR Text |
Show 456 SUMMARY. CHAP, XIII. from presenting a strange difficulty, a~ they .assuredly do on the ordinary doctrine of creation, m1ght even have been anticipated, and can be accounted for by the laws of inheritance. Summary.-In this chapter I have attempted to show, that the subordination of group to group in all organisms throughout all time ; that the nature of the relationship, by which all living and extinct beings are united by complex, radiating, and circuitous lines of affinities into one grand system ; the rules followed and the difficulties encountered by naturalists in their classifications; the value set upon characters, if constant and prevalent, whether of high vital importance, or of the most trifling importance, or, as in rudimentary organs, of no importance ; the wide opposition in value between analogical or adaptive characters, and characters of true affinity; and other such rules ;-all naturally follow on the view of the common parentage of those forms which are considered by naturalists as allied, together with their modification through natural selection, with its contingencies of extinction and divergence of character. In considering this view of classification, it should be borne in mind that the element of descent has been universally used in ranking together the sexes, ages, and acknowledged varieties of the same species, however different they may be in structure. If we extend. the use of this element of descent,-the only certainly known cause of similarity in organic beings,-we shall m:de:stand what is meant by the natural system: It IS gen alogical in its attempted arrangement, with the grades of acquired differenc~. marked by the terms varieties, species, genera, fam1hes, ~rders, a~d cl~sses. On this same view of descent with modrficatwn, all the great facts in Morphology become intelligible,- CHAP. XIII. SUMMARY. 457 hw hethl er we look to the same patte rn dI' sp1 a yed I.n the ?mo ogous o~gans, to whatever purpose applied, of the diffi tr ent sdp emes of a class '. or to the h omo 1o gous parts co~s ructe on the same pattern in each indiv'd animal and plant. I ual On t~e principle of successive slight variations, not nec.e sds arfi ly or generally supervening at 1 1 . .{.' . . a very ear y per1·o d o 11e, and bmng mherited at a correspond I' ng peno ' we can understand the great leading facts in Embryology ; namely, the resemblance in an individual embryo of the homologous parts whi h h t d ·n b ' c w en ma. ure WI ecome widely different from each other In structure and function· and the rese bl . d . .(!£' • ' m ance In Iuerent spemes of a class of the homolog t h ous pars or organs, t ough fitted in the adult memb .{.' d'.(!£' ers 10r pur-poses as Iuerent as possible. Larvoo are t' b h . ac IVe em-ryos, w wh have become specially modified I·n . I t' t ? th · h b' . Ie a Ion eu . a I~s of ~Ife, through the principle of modifica-tiOns be.In~ Inhented at corresponding ages. On . this same pnnmple-and bearing in mind that wh d . . . ' en organs a~e re uced In size, either from disuse or selection it will generally be at that period of life when the b .' has to provide for its own wants, and bearing in ~~n~ how strong i~ the principle of inheritance-the occ~rrence of rudimentary organs and their final abortion present t~ us no inexplicable difficulties ; on the con~ trary,, theu presence might have been even anticipated. Th~ Importance of embryological characters and of rudi~entary organs in classification is intelligible, on ~h~ VIew that. an arrangement is only so far natural as It Is genealogwal. F~nally, the several classes of facts which have been consid~red in this chapter, seem to me to proclaim ;o ~l.ainly, that ~he i~umerable species, genera, and amihes of orgamc beings, with which this world is X |