SPOXTANEOUS GENERATION AND EVOLUTION : CONCLUSION 389 



a relatively high stage of organization, which must have been pre- 

 ceded liy a very long series of ancestoi-s of which no trace has been 

 preserved. The whole basal portion of the animal genealogical tree, 

 from the lowest forms of life at least up to these primitive Crusta- 

 ceans, the Trilobites, lies buried in the deepest sedimentary rocks 

 raised from the sea-floor, the crystalline schists, in which it is 

 unrecognizable. Enormous pressure and, probably also, high tem- 

 perature have destroyed the solid parts as far as there were any, and 

 the soft pai-ts have only left an occasional impression even in the 

 higher strata. 



Thus enormous periods of time must have elapsed from the 

 beginning of life to the lapng down of that deepest ' Palaeozoic ' 

 formation, the Cambrian, for not only does the whole chain which 

 leads from the Biophoridfe to the origin of the first unicellulars fall 

 within this period, as well as the evolution of these unicellulars them- 

 selves into their different classes, and their integration into the Gist 

 multicellulars, but also the evolution of these last into all the main 

 branches of the animal kingdom as it is now, into Sponges, Starfishes, 

 and their allies. Molluscs, Brachiopods, and Crustaceans, for all these 

 branches appear even in the Cambrian formation, and we may 

 conclude that the worms also, most of which are soft and not Kkely to 

 be preserved, were abundantly present at that time, since jointed 

 animals like the Crustaceans can only have arisen from worms. 

 Moreover, we have every reason for the assumption that Ccelenterates 

 also, that is to say polyps and medusoids, lived in the Cambrian seas, 

 because their near relatives with a soKd skeleton, the corals, are 

 represented in the formation next above, the Silurian. The same 

 is trae of the fishes, of which the first undoubtedly recognizable 

 remains, the spines of sharks, have been found in the Silurian. 

 These two presuppose a long preparatory history, and thus we come 

 to the conclusion already stated, that aU the branches of the animal 

 kingdom were already in existence when the earth's crust shut up 

 within itself the first records available for us of the ancestors of our 

 modem world of organisms. 



Of course at that time the higher branches had only been 

 represented by their lower classes, and this is true especially of 

 vertebrates, so that, from the laying down of the Cambrian strata 

 to the modem world of organisms, a very considerable increase of 

 complexity in structure and an infinite diversifying of new groups 

 must have taken place. Amphibians do not appear to have been 

 present in Cambrian times : reptiles are represented in the Carboni- 

 ferous strata, but only appear in abundance in Secondary times; 



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