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Show 396 DARWINISM CI!AI'. about twenty genera.. Nevertheless, a great gap still exists between these mammals and those of the Tertiary strata, since no mammal of any kind has been found in any part of the Cretaceous formation, although in several of its subdivisions abundance of land ph.nts, freshwater shells, and air-breathing reptiles have been discovered. So with fi hes. In the h . t century none lmd been obtained lower than the Carboniferous forma.tion : thirty years later they were found to be very abundant in the Devonian rocks, and later stlll they were discovered in the Upper Ludlow and Lower Ludlow bed:s of the Silurian formation. \V e thus see that such sudden appearances are deccpti ve, and are, in fact, only what we ought to expect from the known imperfection of the geological record. The comlitions h vourable to the fossilisation of any group of a.nimn.ls occur comparatively rarely, and only in very limite:..l areas ; while the conditions es ential for their permanent preservation in the rocks, amid all the destruction caused by dcnmlation or meLamorphism, are still more exceptional. And when they :u·e thus preserved to our day, the particular part of the rocks in which they lie hidden may not be on the surface hut 1mric<l down deep under other strata, and may tlm. , cxc pt in Lhe <:asc of mineral- bc:Lring deposits, be altogether out of our reach. Then, again, how large a proportion of the earth consists of wild and uncivilisccl regions in which no exploration of the rocks has been yet made, so that whether we shall find the fossilised remains of any particular group of :mimal::; which lived during a limited period of the earth's hi story, and in a limited area, depends upon at least a fivefold com hi nation of chances. Now, if we take each of these chance separately a. only ten to one against us (and some arc certainly more tha.u this), then the actual cha,ncc against onr find ing the fossil remains, s:Ly of any one order of m:tmmalia, or of land plants, at any particular geological horizon, will be ahont :t hundred thous<tnd to one. It may be said, if the chances arc so great, how is iL thn.t we find such immense numbers of fossil . pecies cxccc<ling in number, in some groups, all tho. e tha.t arc now living? Bnt this is exactly what we should expect, hecn.usc the nnmhrr of species of organisms that have ever lived upon the earth, since XIII 397 t~e earliest geological times, will r 1 - - t1mes greater than those no' . P. 0 )ably be many hunched knowledge. and hence the v e~IStlllg of which we have any l . ' en01 mous o-aps and h . geo ogiCal record of extinct fo . · b < c asm m the Yet, notwithstanding these ~:s JS n.ot to be wondered at. evolution is true, there ouo·ht ts~s m our knowledge, if pro~re~sion in all the chief tb es o~ lif:ve been, _on the whole, specmhsed forms should hav~~ . · The h1ghcr and more lower and more generalised fo~me. ~nto ;~stence later than the the portions we possess of th:~h at . owever. fragmentary earth, they ought to show us broad~ et tr ce of hfc upou the evolution has taken place \V h Y hat such a progre:s i ve groups, already referred. to e ~ve seen that ~n some special visible and we w1·11 ' sue a progresswn is clearly . ' now cast a hasty o·l . . senes of fossil forms in orde. t . b a?c~ over the ent1re manifested by them 'as a wh~le.o see Jf a Similar progression is The P1·og?·essive De·velopment or Pla t E . f 'J n s. ver smce ossil plants have been collected and . broad fact has been apparent that the l studwd, the the Coal formation-were m . I ear y plants-those of Tertiary deposits the hio-h flam Y. cryptogamou ' while in the intermediate secondary be;~chm~~rmg plants prevailed. In the coniferre-formed a prominent ~rtgymnospcr~s--:cycads and these have usually been held t~ b of k~he vegetati~n? and as between the flowerless a d fl e. a mel of transitiOn form . n owermo- plants th 1 . successiOn has always, broad] ~ ' . e geo og1cal with the theory of evolution. y B~p~~~ng,. been m accordance were very puzzlino- Th h' h y thJs, however, the facts ~nd eq uisetacere~~ppear:d l~uJ~!~~~·ypto~~m~-ferns, lycopo.cls, m the Coal formation at whi h y,danh m Im~ensc profuswn ment they have neve; since s c pcriO t. ey attamed a developthe highest plants-the d' tu~p~ssed or even equalled; while angiosperms-which now 1~~ y ethon~u~k'tnd monocotyledonous the world and exhibit th rm e u of the vegetation of form and' structure were ealmos~ wonderful ~odifica.tioHs of period, when they s~ddenl mos ~n~mown till the Tertiary for the most part under tleappeare m .full development, and, During the l~tter half o~ame genenc forms a.s now exist. great additions have bee . dthet present century, however, n mel e o our knowledge of fossil |