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Show 404 DARWINISM CHAP. in the Oolite; while ants, representing the highly .specialised Hymenoptera, have occurred in the Purb~~k and Lias. . This remarkable identity of the famihes of very anCient with those of existing insects is q~it~ comparable with t~c apparently sudden appearance of existmg genera ?f trees m the Cretaceous epoch. In both cases we feel certam that we must go very much farther back in order to find the ancestral forms from which they were developed, . a~d tha.t . at any moment some fresh discovery may revolutwmse our Ideas as to the antiquity of certain groups. S~ch a discovery was made while Mr. Scudder's work was passmg through the press. Up to that date all the exist.ing orders of true insects appearc<~ to have orio-inated in the Tnas, the alleged moth and beetle of the Coal fo~mation having been incorrectly determined. But now undoubted remains of beetles have been found ht the Uoal mea~ures of Silesia, thus supporting the interpretation of the borin()"s in carboniferous trees as having been made by insects of thi~ order, and carrying back this highly spe.ciali. e(l form of insect life well into Palroozoic times. Such a discovery rcll(ler. all speculation as to the origin of true insects prm~1a tnre, because we may feel sure that all the other orders of msccts, except perhaps hymenoptera and lepidoptera, were contemporaneous with the highly specialised beetles. The less highly organised terr~strial arthropo(h-the Arachnida and Myriapoda-are, as nught be expected, mnch more ancient. A fossil spider has been found in the Carhouiferous, and scorpions in the Upper Silurian rocks of Scotlnml, Sweden and the United States. Myriapoda have been found abunda~tly in the Carboniferous and Devonian formn ti~ns; but all are of extinct orders, exhibiting a more gcnerahsed structure than living forms. Much more extraordinary, however, 'is the presence in the Palreozoic formations of ancestral forms of true insecLs, tenne<l by Mr. Scudder Palreodictyoptera. They consi. t of ge.uern1- ised cockroaches and walking-stick insects (Orthopteroulea); ancient mayflies and allied forms, of which there are six families and more than thirty genera (N europteroidea.) ; three genera of Hemipteroidea resembling :various Homoptera and Hemiptera, mostly from the Carbomferous formatiOn, a .few from the Devonian, and one ancestral cockroach (Palreobla.ttma) XII[ TilE GEOLOGICAL EVIDENCES OF EVOLUTION 405 from the Middle Silurian sandstone of France. If this occurr nee of a true hexapod insect from tho Middle Silurian be really established, taken in connection with the welldefi. ned Coleoptera from .the Carboniferous, the origin of the entire. group of terr.estnal art~u·opoda. is necessarily thrown back mto the Cambnan epoch, lf not earlier. And this cannot be considered improbable in view of the hi ()"hly differentiated land plants-ferns, equisetums, and lycopods~in the Middle or Lower ~!lur!an, and. even a conifer (Cordaites Robbii) in the Upper S1ltman; while the beds of graphite in tho Laurentian were probably formed from terre. trial vegetation. On. the whole, the~, we ~ay affirm that, although the ~eologiCal 1:ecord of ~he msect hfc of the earth is exceptionally Imperfect, It y~t ?ccidedly supports the evolution hypothesis. The mos~ pecmhsed order, ~epidoptera, is the most recent, only datmg back to the Oohtc ; tho Hymenoptera, Diptera, a.nd Homoptera go as far as th .Lias; while the Orthoptera and N curoptera extend to the Tnas. The recent discovery of Coleop.tera .in. the Carboniferous shows, however, that the precedmg hm1ts are not absolute, and will probably soon be o~erpas~ed. Only the more generalised ancestral forms of wmged I.nsects have been traced back to Silurian time, and alo~1g With them the less highly organised scorpion j facts whiCh serve to show us the extreme imperfection of our ~no~vleclge, and indicate possibilities of a world of terrestrial hfe m the remotest Palreozoic times. Geological Succession of Vertebrata. . T~e lowest form~ of vertebr~tes are the fishes, and these appear first m the ge?log1cal record m tho Upper Silurian formation. The most ancient known fish is a Pteraspis, one of the bucklered. ganoids or plated fishes-by no means a very low type -al~Ied to the sturgeon (Accipenser) and alligator- gar (Lepidosteu?), but, as a group, now nearly extinct. Almost equally anc1~nt are the sharks, which under various forms still abo~nd m our seas. W c cannot suppose these to be nearly the earliest fishes, especially as the two lowest orders now represented by the Amphioxus or la.ncelet and the l::tmr~reyhavc not y~t been found .fossil. The ganoids were great!; developed m the Devoman era, and continued till the |
