ABSENCE OF INVERSION 271 



Let us, he says, suppose with Professor Huxley, that a 

 Silurian, Devonian, and Carboniferous fauna were simul- 

 taneously in existence in three different geographical 

 regions. They may be represented by the letters S.D.C. 

 arranged in any order as in the diagram (Fig. 84) . If, now, 

 these three faunas, preserved in the fossil state, are to 

 succeed one another vertically in the same order in the 

 sediments deposited in the three regions, then it is 

 necessary that their migrations from area to area should 

 take place in one direction only, as indicated by the 

 arrows, so that D shall come to be found over S and C 

 over D. If migrations should occur in contrary directions, 

 as S towards D or D towards C, then beds containing 



FIG. 84. 



remains of Silurian organisms would be found overlying 

 others containing Devonian fossils, and similarly Devo- 

 nian beds would be found above Carboniferous; i.e., there 

 would be no homotaxis, or the universally observed order 

 of succession would be inverted : and since not a single 

 case of such inversion has been shown to exist, Heilprin 

 concludes that the simultaneous existence of three such 

 different faunas as the Silurian, Devonian, and Carboni- 

 ferous is, to say the least, very unlikely. 



The argument is specious, and may contain some germ 

 of truth, but it is not conclusive ; for causes may exist 

 which do actually determine that movement of migration 

 in one direction which at first sight seems to be so im- 



