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Show 406 DARWINISM CHAP. Cretaceous when they gave way to the true osseous fishes, which had 1 first appeared in the Jurassic period, .and have continued to increase till the present day. This much later appearance of the higher osseous fishes is quite in accorda,nce with evolution, although some of th? very lowes~ forms, the lancelet and the lampreys, together w1th the archa1c ceratodns, have survived to our time. The Amphibia, represented by the extinct labyrintho(lons, appear first _in the Car~oniferous rocks, and t.hese peculiar for~m; became extmct early m the Secondary penod. The labynnthodons were, however, highly specialised, and do not at all indicate the oriain of the class, which may be as ancient a the lower forms of flshes. Hardly any recognisable remains of onr existing groups-the frogs, toads, and. sal~m~nders-are found before the Tertiary period, a fact w h1ch mdicates the extreme imperfection of the record as regards this class of animal . True reptiles have 11ot been found till we reach the Permian where Prohatteria and Proterosaurus occur, the former closely allied to the lizard-like Sphenodon of New Zeahtml, the latter having its nearest allies in the same gronp of reptiles-Rhyncocephala, other forms of which occur in the Trias. In this last-named formation the earliest crocodilvsPhytosaurus (Belodon) and Stagonolepis occur, as well a~ the earliest tortoises-Chelytherium, Proganochelys, and PHephoderma. 1 Fossil serpents have been first found in the Cretaceous formation, but the conditions for the preservation of these forms have evidently been unfavourable, and the n•curd is correspondingly incomplete. The marine Plesiosami and Ichthyosauri, the flying Pterodactyles, the terrestrial Ignanodon of Europe, and the huge Atlantosaurus of Coloradothe largest land animal that has ever lived upon the earth~all belong to special developments of the reptilian type ,,·hich flom·ished during the Secondary epoch, and then became extinct. 1 For the facts as to the early appearance of the above name<l groups of reptiles I am indebted to Mr. R. Lyclekker of the Geological Departnwnt of the Natural History Museum. 2 According to Professor Marsh this creature was 50 or 60 feeL long, au<l when erect, at least 30 feet in height. It feel upon the foliage of the mountain forests of the Cretaceous epoch, the remains of which arc p re~erv etl with it. Xlll THE GEOLOGICAL EVIDENCES OF EVOLUTION 407 Birds are amona the rarest of fossils due no doubt to their aerial habits ~·emov~ng them from the m:<linnry d;ngers of flood, bog, .or ICe whiCh overwhelm mammals and reptiles, and a,l o to thmr small speci~c gravity which keeps them floating on the surface of .water till ~levoured. Their remain were long confined to Tertiary depo ·1ts, where many livin cr crenera and a few extinct forms have b en found. The o~l; birds yet k~1own from the older rocks are the toothed bird. (Odontor~ uthcs) of t~e .Cretacc~u_s beds of the United States, belongmg to two ~Isti.nct famll10s and many genera; a penguin-like form (Enahorms) from the Upper Greensand of Cambrido-e · and. the well-kn.own }long-tailed ~rch::e~pteryx from the up£e; Oob.te of Bavaria. 'I he r?conlis thus Imperfect and fragmentary m the extreme ; but It yet shows u , in the few bird. discovered in the older rocks, more primitive and rrencralised types, while the Tertiary birds bad already become ~peciali sed like those living, and had lost both the teeth and the lona vertebral tail, which indicate reptilian affinities in the em·lie~ ages. Mammalia have been found, as already stated, as hr back as the Trias formation, in Europe in the United States and in South Africa, all being very small, and belono-ino· either to the Marsupial order, or to some still lower b an~l more generalised type, out of which both Marsupials and In. ectivora were developed. Other allied forms have been fonnd in the Lower and Upper Oolite both of Emope and the United States. But there is then a great gap in the whole Cretaceous formation, from which no mammal ha: been obtninecl althouuh hoth in the \V calden and the Upper Cha,lk in Enro~c, and in the Upper Uretaceons deposits of the United , 'tates an abundant and well -pre. erv d t rrestrial flora has been discovered. \Vhy no nwmmnL have left their remains here it is impo . ible to say. \Ve can only nppose that the limited areas in which land plants have hcen so almnclantly pre erYcd, did not present the cond iti o n ~ w11ich nrc needed for the fo. silisation and preservation of mammalian remains. When we como to the Tertiary formation, we find mammals in abundance ; but a wontlerful ch:wge has taJwn place. The obscure early types have eli appeared, and we di cover in their place a whole series of forms belonging to existing orders, |