820 GROUPS OF ALLIED SPECIES 



tracts of land have existed, subjected, no doubt, to great 

 oscillations of level, since the Cambrian period. The colored 

 map appended to my volume on Coral Reefs led me to con- 

 clude that the great oceans are still mainly areas of sub- 

 sidence, the great archipelagoes still areas of oscillations of 

 level, and the continents areas of elevation. But we have 

 no reason to assume that things have thus remained from 

 the beginning of the world. Our continents seem to have 

 been formed by a preponderance, during many oscillations 

 of level, of the force of elevation. But may not the areas of 

 preponderant movement have changed in the lapse of ages ? 

 At a period long antecedent to the Cambrian epoch, conti- 

 nents may have existed where oceans are now spread out, 

 and clear and open oceans may have existed where our con- 

 tinents now stand. Nor should we be justified in assuming 

 that if, for instance, the bed of the Pacific Ocean were now 

 converted into a continent, we should there find sedimentary 

 formations, in recognizable condition, older than the Cam- 

 brian strata, supposing such to have been formerly deposited ; 

 for it might well happen that strata which had subsided 

 some miles nearer to the centre of the earth, and which had 

 been pressed on by an enormous weight of superincumbent 

 water, might have undergone far more metamorphic action 

 than strata which have always remained nearer to the sur- 

 face. The immense areas in some parts of the world, for 

 instance in South America, of naked metamorphic rocks, 

 which must have been heated under great pressure, have 

 always seemed to me to require some special explanation ; 

 and we may perhaps believe that we see in these large areas 

 the many formations long anterior to the Cambrian epoch in 

 a completely metamorphosed and denuded condition. 



The several difficulties here discussed, namely, that, though 

 we find in our geological formations many links between the 

 species which now exist and which formerly existed, we do 

 not find infinitely numerous fine transitional forms closely 

 joining them all together, the sudden manner in which several 

 groups of species first appear in our European formations, 

 the almost entire absence, as at present known, of formations 

 rich in fossils beneath the Cambrian strata, are all undoubt- 

 edly of the most serious nature. We see this in the fact 

 that the most eminent pa.aeontologists, namely, Cuvier, 

 Agassiz, Barrande, Pictet, Falconer, E. Forbes, etc., and all 

 our greatest geologists, as Lyell, Murchison, Sedgwick, etc., 

 have unanimously, often vehemently, maintained the immu- 



