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Show 338 GEOLOGICAL SUCCESSION. CHAP. X. species of the two countries could not have foreseen this result. . A . . · ts that ancient animals resemble to n. I gass1z InSIS . 1 f cer t ai·n ex t ent the err1bryos of recent animas .o t 11 e. same cI asses; OI• that the ba eological successwn of f extinct forms is in some degree parallel to the embryo- } logical development of recent forms. I must follo;v Pictet and Huxley in thinking that the truth of thiS doctrine is very far from proved. ! et I fully expect t? see it hereafter confirmed, at least In regard to subordinate groups, which have branc?ed off !rom ~ach otl:er \ within co1nparatively recent. times. :E or this doctnne of Agassiz accords well with the theory of natural selection. In a future chapter I shall attempt to show that the adult differs from its embryo, owing to varia-tions supervening at a not early age, and being .inh~rited at a corresponding age. This process, whilst It leaves the embryo almost unaltered, continually adds, in the course of successive generations, more and more difference to the adult. Thus the embryo comes to be left as a sort of pict?re, preserved by nature, of the ~nci~nt and less modified condition of each animal. This VIew may be true, and 1 yet it may never be capable of full proof. Seeing, for instance, that the oldest known mammals, reptiles, and fish strictly belong to their own proper classes, thou~h some of these old forms are in a slight degree less cbstinct from each other than are the typical members of the same groups at the present day, it would be vai~ to look for animals having the common embryological character of the Vertebrata, until beds far beneath the { lowest Silurian strata are discovered- a discovery of which the chance is very small. . On the Sueeessio'n of tlte same Types within the same CHAP. X. SAME TYPES IN SAME AREAS. 339 areas, during the later tertiary periods.-l\fr. Clift many years a?o showed that the fossil mammals from the Au~trahan caves ":ere closely allied to the living marsupi~ ls of .tha.t continent. In South America, a similar :elatlon~hip ~s ~anifest, even to an uneducated eye, In the gigantiC pieces of armour like those of the armadillo, found in several parts of La Plata ; and Professor Owen has ~hown in the most striking manner that most of the fossil mam~als, buried there in such nlunbers, ar? r.elated to South American types. This relationship Is even more clearly seen in the wonderful collection of fossil bones made by MM. Lund and Clausen in the caves of Brazil. I was so much impressed with these .facts that I strongly insisted, in 1839 and 1845 on this "law of the succession of types,"-on "this won~ derful relationship in the same continent between the dead and the living." Professor Owen has subsequently extended. the same generalisation to the mammals of the Old. World. We see the same law in this author's restorations of the extinct and gigantic birds of New Zeal~nd. We see it also in the birds of the caves of Brazil. Mr. Woodward has shown that the same law ~olds good with sea-shells, but from the wide distributiOn of most genera of molluscs, it is not well displayed ~y them. Othe: cases could be added, as the relation etween the extinct and living land-shells of Madeira· and between the extinct and living brackish-water shell~ of the Aralo-Caspian Sea. . Now what does this remarkable law of the succesion of the same types within the san1e areas mean ? e wo~ld be a bold man, who after comparing the pre~ ent chmate of Australia and of parts of South America nder the same latitude, would attempt to account 011 ~~e . o~e ~and, by dissimilar physical conditions for' the ISsimilarity of the inhabitants of these two continents . ' Q 2 |
