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Show 376 DARWINISM OHAP. through which animals have. passed, it ~ill naturally be. ex-t d that we should find Important evidence of evolution. pee e . h . l l W e sh ou ld hope to learn the steps by whJC some ISO atec 11. d · forms have been connected with their near.est a ICS, an m any ca.ses to have the o-aps filled up whiCh now separate m, o . I th genus from genus, or species fro~n specws. n some cases esc expectations are fulfilled, b~t m many. other case~ we seek in vain for evidence of the lnnd we des11·e; and th~s ~bsence of evidence with such an apparent wealth of matenal IS h~ld by many persons to throw doubt on the theory of evolutiOn itself. They urge, with much appearance of reason, that all the arguments we have hitherto adducec~ fall. shor~ of demonstration, and that the crucial te. t consists m. beu:g able. to show, in a great number of cases, those connectmg h~ks wh1 ~h we say must have existed. Ma~y of the gaps t~at still remam are so vast that it seems incredible to these wnters that they could ever have 1 een filled up by a close succession of speci eR, since these must ha,ve spread over so many ages, and have existccl in such numbers, tha.t it seems impossible to account for their totu.l n.bscncc from deposits in which great numb rs of species belono·in()' to other groups are preserved and hn,vc been discovered~ fn order to appreciate the force, or wcalme s, of these objections, we must inquire i~to the character a.nd completeness of that record of the .past hfe of the earth wlnch geology has unfolded, :wd ascertam the. ?aturc and amount of the evidence which, under actual conditiOns, we may expcd to find. The Nztrnber of known Species of Extinct A nirrwls. vVhen we state that the known fossil mollusca are considerably more numerous than those which now l_ive on the earth, it appears at first sight that our knowledge IS v~ry complete, but this is far from being the case. The species have been continually changing throughout geological time, and at each. period have probably been as numerous as they are n~~· . If we divide the fossiliferous strata into twelve great clivi lOllS -the Pliocene, Miocene, Eocene, Cretaceous, Oolite, Lias, Trias Permian, Carboniferous, Devonittn, Siluriau, and Cambrian' -we find not only that each has a very distinct :mel char;cteristir. molluscan fauna, but that the different . nh- XIII THE GEOLOGICAL E\'IDENCES OF EVOL TION 377 divisions often present a widely different series of species ; Ro that although a certain number of species arc common to two or more of the great clivi ions, the totality of the species that have lived upon the earth must be very much more than twelv.c times-perhaps even thirty or forty times-the nm~bcr now living. In lil~e manner, although the species of foss~l mam1:1al. now recogmsed by more or less fragmentary fos. 1l rcmams may not be much less numerous than the living species, yet the duration of existence of these was comparatively so short that they were almost completely cha~1gc<l, perh~ps. six o: . even times, during the Tertiary pcnod; and th1s IS certamly only a fragment of the O'Colorrical time during which mammalia existed on the globe. 0 0 There is also reason to believe that the higher animals were much more abundant in Sl)Ccies durino· })U t ()'eoloo-ical . b b b ep~chs than now, owmg to the greater equability of the climate whJCh rendered even the arctic regions as habitable as the temp rate zones are in our time. _The sa~e .equ~ble climate would probably cause a more umform (hstnbutwn of moisture, and render what are now desert regions capable of supportirw abundance of animal life. This is i.ndicated by the number a1~d variety of the species of lar~c ammals that have been found fossil in very limited areas whJCh they evidently inhabited at one period. M. Albert Gauch·y found, in the deposit. of a mountain stream at Pikcrmi in Greece, an abuncbnce of large mammalia such as are nowhere to be found living together at the present time. A~ong them were t-wo species of Mastodon, two different rhmoceroses, a gigantic wild boar, a camel and a ()'iraff'c larg~r than those now living, several monkey , car~ivora rangmg ~rom martens and civets to lions and hyrenas of the largest ~Izc, numerous antelopes of at least five distinct genera, and bcs1dcs these many forms altogether extinct. Such were the great herds of Hipparion, an ancestral form of horse · the Helladotherium, a huge animal bigger than the ()'iraffc ~ the Ancylotherium, one of the Edentata · the hu()'e Di~rotherium · ) b ) the Aceratherium, allied to the rhinoceros; and tho monstrous Chalicotherium, allied to the swine and ruminants, hut as large as. a rhinoceros; ancl to prey upon these, the gr at Machmrodus or sabre-toothed tiger. And all these remains were |