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Show II 'I 238 EXPI.AN ATIONS. conditions, or resultincr from equality of grade in the scale. True affinities~and these are the affinities of genealogy-are not to be looked for horizontally amongst ordt ':'s, but vertically, from an order in one class to the corresponding order in the class next higher. Generally, the first and lowest forms of the orders in a class are marine, and often these are of comparatively large size. We usually see in them a vestige of the essenllal charac .. ters of the 'class next below. Thus, the perennibranchiate batrachia in their order, the ichthyosauri in the series of crocodilia, and the divers among birds, all exhibit an affinity to fish. The cetacea and phocidre, which I regard as the immediate basis of the pachydermata, carnivora, and other orders of terrestial mammals, ought, according to this view, to show an alliance to the reptiles; and such a connection does exist between the cetacea and certain marine saur.ia, but from the general extinction of the r.aarine reptiles, the linking of the mammals to that lower class is less clearly seen than might be wished. It must be kept in view that only an outline of the progress of the animal kingdon1 is here designed. Excep~ tions as to the course which· tlevelopment has taken appear to be by no means few; leading to the idea that the grades of organization are not determinate in this respect, but may be reached by steps of unequal length. Thus, for example, the marsupials appear very clearly a development from certain birds ; probably the rodent and edentate orders are derived through the same channel. From the approach made by certain of the reptilia to birds, we may surmise that there also there are excep· tions to the rule. In short, the progress of animality in the different stirpes has been attended by peculiarities which evidently affix peculiar characters to each, and n1ake the idea of a difference in time not only probable, but unavoidable. Regarding the animal kingdoll!- sill!-ply ~s ~ combin~t.ion of independent stirpes, each w1th 1ts du~tmct affin1hes, the theory of transrnutation puts on a totally new aspect; so truly is this the case, that transrnutation is hardly any longer a term appropriate to the idea. The difficulty ot supposing such changes as that from the rodent to the rll· minant, or the carnivorous animal to the quadrurnane, vanishes, leaving only t·ransitions from one form ~o another of a series generallp similar-from the aquatic EARLY REPTILIAN FOSSIL:!1. 239 pachydernt, for instance, to the terrestrial, from the otary to the otter, from certain phocce to the bear, and so on. There is a unity in all instances in the moral as well as physical characters of the various members of one stirps; we only see it advancing from low to high characters, just as we see the fcetus of a high animal pa8sing through various inferior stages before it reach its proper mature character. The lines, moreover, being independent of each other, and not quite uniform as to the stages of anirnality through which they pass, it follows that, unless we know of some law governing their different gestative periods, we are not entitled to look for the first occurrence of their various ichthyic, reptilian, and mammalian sections, in any order as regards each other, even though we could be sure (which we are not) that we are surveying a geographical region where they all started fair in the race of progressive organization. Hence it is that, though the batrachia are usually placed by zoologists at the bottom of the list of reptilian orders, I attach little importance to their vestiges being now found so low All that i think we can expect is, tftat, in a particular area where we have reason to believe that the lines have started abreast, they should all reach their various grades nearly about one time, or what may be considered as one time compared with the whole extent of geological chronology. And such appears to be pretty much the case in those regions which geologists have explored. The Edinburgh reviewer will observe that this view of the animal kingdom leaves much of his opposition in a very awkward predicament. He has everywhere assumed that the genealogy of the orders of each class was supposed to be en suite, \\~hic.h it certainly never was .in my book. In the early editions I spoke with diffidence of the course of the supposed development,* because I had not then seen or conceived any arrangement of the animal kingdom which answered to that hypothesis, although I thought proper to attempt to show that the quinarian and c.ircular classification, which I found In vogue at the time when I was writing, did not neces.; arily militate against it. In the third edition, the pres· .,. " ... it does not appear that this gradation passes along ~one line, on which every animal form can be, as it were, sfrung; there may he branching or double lines at some places," &c.-Ytstiges, ls~ ed., p. 130 |