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Show GEOLOGICAL SUCCESSION. [Cn.AP. X. 290 same general characteristics. This is represented in the diagram by the letter F 14 • All the many forms, extinct and recent, descende~ from A, make, as before remarked, on~ or~er ; and. this order from the continued effects of extinction and divergene~ of character has become divided into several subfamilies and famili~s, some of which are supposed to have J.leriahed 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 h If i! • t th 1 r. 10 /8 s 0 ot er. , 10r 1ns ance, e genera a, a, a , , m, m, rn}, 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 senera, which thus linked the living genera of thr~e ~amihes together, .interme~iate in character, would be JUStified, as they are 1ntern1ediate, 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 formations-for .instance, above No. VI.-but none from beneath this line, then only the two families on the left hand (namely, a 14 , &c., and b14 , &c.) would have to be united into one family ; and the two other fainilies (namely, a14 to / 14 now including five genera, and o14 to m14 ) would yet remain distinct. These two families, however, would be less distjnct from each other than they were before the discovery of the fossils. If, for instance, we suppose the existing genera of the two families to differ from each other by a do~en characters, in this case the genera, at the early per1od n1arked ~I., would differ by a lesser number of cha~acters; for at this early stage of descent they have not diverged in character from the common progenitor of th~ orde:, pearly so much as they subsequently diverged. TI1us 1t CHAP. X.] AFFINITIES OF EXTINCT SPECIES. th t . . 291 c~mes a ancient and extinct . . sligl~t de~ree intermediate in g;~re:a ai e often ln some modified descendants or between th . ctefl between their In nature the cas~ will be f: mr co ateral relations. is represented in the diagram . afo~~he complicat~d than been more numerous they will h J gr01fos will have une.qual lengths of time, and wrlre hen ured or ext.renle~y varwus degrees As we ave been modified In the geological r~cord and l~s~e~s only the last volume of we have no right to' ex ect a In a v~ry broken condition, fill up wide intervals in £he ~a~x~eft In very rare cases, to distinct families or orders A~lathystem, and thus unite expect, is that those ro~ s h' at we have a right to geological periods unde~go!e ~w ~ch ~ilie ~ithin known the older formations make som uc l·m{ cation, should in other; so that the older m be s I~ 1t approach to each each other in some of the. e\n ers s lould differ less from members of the same ro~ cs 1.aracters .than do the existing evidence of our best ffalreEnt tn~ ~his by the concurrent be the case. 0 ogis s seems frequently to Thus, on the theory of d . main facts with res ect t escent With modification, the extinct form~ of lif~ to 0 ~he hmutual affinities of the seem to me ~xplained in a ::~isf~;t er and to living forms, are wholly Inexplicable on an othr! n;anner. And they On this same theor it . y . el Vlew. ~eat period in the el~th'~s ~yide~t t~at the fauna of any ln gener~l character between ths~oi ~ :Whll be Intermediate whwh succeeded it Th a w .1c preceded and that sixth ~reat stage of. d us,tt?e species which lived at the fi ed o spri. ng of thosee swcehnic hI n1 ' thed diagr am are t h e modi.- are the parents of those h. h~ at the :fifth stage, and fied at the seventh t w Ic ecame still more modi-to be nearly interme~i:fe ~ hhce they could hardly fail of life above and b 1 e In c aracter between the forms the ~ntire extinction eofw. We mus.t, however, allow for comlng in of quite new f~~sprec~ding forl?s, and for the !arge amount of modific t' ~y ~mmigratlon, and for a Intervals between th a Ion, . unng the long and blank these allowances the£ succeflve formations. Subject to ' auna o each geological period un- |