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Show 1 ~ { 474 RECAPITULATION. CHAP. XIV. and therefore these same characters wou~d be more likely still to be variable t~an the genenc characters which have been inherited without change for an enor ·mous period. It is inexplicable on the theory of creation why a part developed in a very unusual manner in any one species of a g~nus, and therefore, a~ we may naturally infer, of great Impo~tance to the speme~, shoul? be eminently liable to vanation; but, on my VIew, thJs part has undergone, since the several species branched off from a common progenitor, an unusual amount of variability and modification, and t~erefo::e we might expect this part generally to be still vanable. But a part may be developed in the most unusual ma~ner, like the wing of a bat, and yet not be more vanable than any other structure, if the part be comn1on to many subordinate forms, th~t is, .if it ha~ b~en inherited for a very long period; for tn this case It will have been rendered constant by long-continued natural selection. Glancing at instincts, marvellous as some are, they offer no greater difficulty than does corporeal structure on the theory of the natural selection of successive, slight, but profitable modifications. We can. thus un~ersta~d why nature moves by graduated steps In ~ndowing ~Ifferent animals of the same class with thmr several Instincts. I have attempted to show how much light th: principle of gradation throws on the. admirable architectural powers of the hive-bee .. ~abi~ n~ doubt som~· times comes into play in modifying Instincts ; but It certainly is not indispensable, as we see, in ~he e~se of neuter ins cts which leave no progeny to Inherit the effects of long~continued habit. On the view of all the species of the same genus having descen~ed from a common par nt, and having inherited ~uch In ?ommon, we can understand how it is that alhed speCies, w~en placed und r considerably different con dI·t ·w ns of hfe' CHAP. XIV. RECAPITUI.~ATION. 475 yet should follow near~y the sa.me instincts ; why the thrush of South Amenca, for Instance lines he t W.i t h mu d lI'k e our British species. O' n the vire wn eos f instin?ts having been slowly acquired through natural selection we need not marvel at some instincts being apparently not perfect and liable to mistakes, and at many instincts causing other animals to suffer. If speeies be only well-marked and permanent varieties, we can at once see why their crossed offspring should follow the same complex laws in their degrees and kinds of resemblance to their parents,-in being absorbed into each other by successive crosses, and in other such points,-as do the crossed offspring of acknowledged varieties. On the other hand, these would be strange facts if species have been independently created, and varieties have been produced by secondary laws. If we admit that the geological record is imperfect in an extreme degree, then such facts as the record gives, support the theory of descent with modification. New species have come on the stage slowly and at successive intervals; and the amount of change, after equal intervals of time, is widely different in different groups. The extinction of species and of whole groups of species, which has played so conspicuous a part in the history of the organic world, almost inevitably follows on the principle of natural selection ; for old forms will be supplanted by new and improved forms. Neither single species nor groups of species reappear when the chain of ordinary generation has once been brok_en. The gradual diffusion of dominant forms, with the slow modification of their descendants, causes the forms of life, after long intervals of time, to appear as if they had changed simultaneously throughout the world. The fact of the fossil remains of each formation being in some degree intermediate in character between the |
