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Show CHAP. VI. ICULTIES ON THEORY. 172 DIFF the eye of which we hardly d f 1 t ·ucture as ' . ? such won er u s r d he inimitable perfection . as yet fully understan t . ed and modified through . t' ts be acq u1r Thirdly, can Ins Inc . h 11 we say to so marvellous natural selecti.o n ? Wh.a th s1 a ds the bee to make ce 11 s, an instinct as th~t whw t ~~pated the discoveries of h. h have praetwally an ICI w lC • • ? Profound mathematiCians . t .{!or species when crossed, 11 h an we accoun 11 ' Fourt 1 ~' ow c d . sterile offspring, whereas, being stenle and pro ucin~ their fertility is unimwhen varieties are crosse ' paiTrehde ?tw o first head s sh a1 1 b e here discussed-Instinct and Hybridism in separate chapters. 0 th absence or ran' t Y O'J'. f transitional varie•t ies.-f n e 1 b the preservatiOn o As natural selection acts sole y y .{! ·n tend in a . · ch new 1orm Wl Profitable modificatiOns, ea 1 f d finally to . t t ke the pace o, an fully-st?cked .country o ~ d arent or other less-exterminate, Its o':n le: I:1;o:~m~s into competition. favoured forms with w c 1 t' wi'll as we have . · d atural se ec 10n ' Thus extinctiOn an n . f look at each Sl)ecies seen, go hand in hand. Hence, I we form both the as descended from some other unk~o':n . ' enerally t d all the transitional vaneties will g paren an rocess of forma-have been exterminated by the very p tion and perfection of the new form. . t. al forms But as by this theory innumerable transi Ion b dded must h' ave eX.I ste d , w h Y d 0 we not finhd themtl em e ? It will in countless numbers in the c~st of :h ~ e~es~ion in the be much more convenient to discuss lS ( . 1 record. chapter on the Imperfection of the ge? o;l~~e answe~· and I will here only state_ th~t I behev 1 ,. less erfect mainly lies in the record being Inco~para~ )t. ~f the than is generally suppose d '. tl: e Im.p en ec t winnh abiting record being chiefly due to organic beings no CHAP. VI. TRANSITIONAL VARIETIES. 173 profound depths of the sea, and to their remains being embedded and preserved to a future age only in masses of sediment sufficiently thick and extensive to withstand an enormous amount of future degradation; and such fossiliferous masses can be accumulated only where much sediment is deposited on the shallow bed of the sea, whilst it slowly subsides. These contingencies will concur only rarely, and after enormously long intervals. Whilst the bed of the sea is stationary or is rising, or when very little sediment is being deposited, there will be blanks in our geological history. The crust of the earth is a vast museum; but the natural collections have been made only at intervals of time immensely remote. But it may be urged that when several closely-allied species inhabit the same territory we surely ought to find at the present time many transitional forms. Let us take a simple case : in travelling from north to south over a continent, we generally meet at successive intervals with closely allied or representative species, evidently filling nearly the same place in the natural economy of the land. These representative species often meet and interlock; and as the one becomes rarer and rarer, the other becomes more and more frequent, till the one replaces the other. But if we compare these species where they intermingle, they are generally as absolutely distinct from each other in every detail of structure as are specimens taken from the metropolis inhabited by each. By my theory these allied species have descended from a common parent; and during the process of modification, each has become adapted to the conditions of life of its own region, and has supplanted and exterminated its original parent and all the transitional varieties between its past and present states. Hence we ought not to expect at the |
