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Show 332 GEOLOGICAL SUCCESSION. fjHAP. X, modified in relation to its slightly altered conditions of life, and yet retain throughout a vast period. the sa~e general characteristics. This is represented 1n the diagram by the letter F 14 • All the many forms, extinct and recent, descende.d from A make as before remarked, one order; and this order from the continued effects of extinction and diver~ence of character, has become divided into severa{' sub-fan1ilies and families, son1e of which are supposed to have perished at different periods, and some to have endured to the present day. By looking at the diagram we can see that if many of the extinct forms, supposed to be embedded in the successive formations, were discovered at several points low down in the series, the three existing families on the uppermost line would be rendered less distinct from each oth er. If, 1.[! 0r I• nstance, the genera a 1, a o, a 1 o, fs , m s, m 6, mg , were disinterred, these three families would be so closely linked together that they probably would have to be united into one great family, in nearly the same manner as has occurred with ruminants and pachyderms. Yet he who objected to call the extinct genera, which thus linked the living genera ofthree families together, intermediate in character, would be justified, as they are intermediate, not directly, but only by a long and circuitous course through many widely different forms. If many extinct forms were to be discovered above one of the middle horizontal lines or geological form.ations -for instance, above No. VI.-but none from beneath this line, then only the two families on the left hand (namely, a14, &c., and b1 \ &c.) would have to be united into one family; and the two other families (namely, a14 t.o jt4 now including five genera, and o14 to m14 ) would yet remain distinct. These two families, however, would be less distinct from each other than they were before the CHAP. X. AFFINITIES OF EXTINCT SPECIES. 333 discovery of the fossils. If, for instance, we suppose the existing genera of the two families to differ from each other by a dozen characters, in this case the genera, at the early period marked VI., would differ by a lesser number of characters; for at this early stage of descent they have not diverged in character from the common progenitor of the order, nearly so much as they subsequently diverged. Thus it comes that ancient and extinct genera are often in some slight degree intermediate in character between their modified descendants, or between their collateral relations. In nature the case will be far more complicated than is represented in· the diagram; for the groups will have been more ·numerous, they will have endured for extremely unequal lengths of time, and will have been modified in various degrees. As we possess only the last volume of the geological record, and that in a very broken condition, w~ have no right to expect, except in very rare cases, to fill up wide intervals in the natural system, and thus unite distinct families or orders. All that we have a right to expect, is that those groups, which have within known geological periods undergone much modification, should in the older formations make some slight approach to each other; so that the older members should differ less from each other in some of their characters than do the existing members of the same groups ; and this by the concurrent evidence of our best palreontologists seems frequently to be the case. Thus, on the theory of descent with modification, the main facts with respect to the mutual affinities of the extinct forms of life to each other and to living forms, seem to me explained in a satisfactory manner. And they are wholly inexplicable on any other view. On this same theory, it is evident that the fauna of any great period in the earth's history will be inter- |
