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Show 408 DARWINISM OliAP. --------------------------- and even sometimes to existing families. Thus, in the Eocene we have remains of the opossum family.; bats apparently b el ongm· g to livinOe' genera.· ' rodents . alhed to ft hde S·o ut1h American cavies and to dormice and sqmrrols; hoo e amma s belonO'iJw to the odd- toed and even-toed groups; and ancestral f~rms of cats, civets, dogs, wit~ a number of more generalised forms of carnivora. Besides thes~ there a,re whn.les, lemurs, and many strange ancestral forms of pro-boscidea. 1 · The O'reat diversity of forms and structures at so remote an epoch would require for their developmep.t an amount ?f t ·me which J'udO'ino· by the changes that have occurred 111 I, 'eb f. hM. ()ther groups, would carry us back ar mto t e esozo1c period. In order to understand why we have no record of these chan()'es in any part of the world, we must fall back upon some e such supposition as w~ made in the case of the dicotyledonous plants. Perhaps, 1~doed, the t:vo cases are really connected, and the upland regwns .of the pnme~al world, which saw tho development of our h1gher vegetatiOn, may have also afforded the theatre for the gradual developm ent of the varied mammalian types which surprise us by their sudden appearance in Tertiary tim~s: . Notwithstanding those irrogulant10s and gaps m the record, the accompanying table, summarising our actual knowledge. of the o·eoloO'ical distribution of tho five classes of vertebrata, b b GEOLOGICAL DISTRIBUTION OF MAMMALIA. Fishes . Amphibia Reptiles Birds . Mammalia -~ E ii3 tJi ::> 2 dc; ~ -~ ·~ 0 ~ ~ c; ~ 0 d 0: ·~ tJi 1l <ll tJi ~ ~ ~ ~ 0 tJi ::> :il ~ C) ~ ~ u ~ 1 For fuller details, see the author's GeograP_hic_al J?ist1'i?utio_n nf "In im cds, and Heilpriu's Ueographical aud Ueoloyicctl Dtstnbutwn oj .rhtwwls. xrrr THE GEOLOGICAL EVIDENCES OF EVOLUTION 409 exhibi~s a steady progre sion from lower to higher types, exceptmg only the deficiency in tho bird record which is ea;s~ly explained. The comparative perfection of type in whiCh ?a.ch of these classes first appears, renders it certain that the ongm of ea.ch and all of them must be soucrht much farther ba.ck than any records which have yet been discovered. The r~s.earche.s ?f palre?ntologists and embryologists indicate a rep~1l~an ongm for b1rds and mammal , while reptiles and ampl11 b1a arose, perhaps independently, from fishes. Concluding Rema?'l.·s. The brief review we have now taken of the more suggestive facts presented by the geological succession of or(J-anic forms i . suffi~ient t? s~ow that rr:ost, if not all, of the . upposecl difficulties whiCh It presents m the way of evolution, are due either to imperfections in the geoloO'ical record itself or to our still very incomplete knowledge of ~vhat is really r~corded in the earth's crust. \V e learn, however, that jnst as discovery progresses, gaps are filled up rtnd clitficn]tios disappear; while, in the case of many individual groups, we have alrea ly obtained all tho evidence of progre sive development tha.t can reasonably be expected. \Ve conclude, therefore, th;tt the geological difficulty has now disappeared; and that this noble science, when properly understood, affords clear and wei()'hty evidence of evolution. e |