EXPLANATIONS. 



of development, each of which is a small advance upon the 



preceaing, and the type of a form thenceforth to continue 

 permanent. Each line stands apaft. It may show shad- 

 ings in a vertical direction, as between its reptilian and itg 

 mammai forms, but no true affinities connecting horizon- 

 tally with the members of other lines. Our critic is here, 

 therefore, completely at fault. I meet him again, how- 

 ever, on special grounds Many of the animals of the 

 tertiary period are of large bulk. We have not only huge 

 examples of the pachyderm order, in which there are still 

 existing many bulky species, but we have equally vast 

 creatures oe.onging to the rodent, the edentate, and 

 other oraers. These huge mammals are, indeed, the sig- 

 nal iorms of this period, the forms by which the whole 

 tertiary system is most distinguished. Now, if we take 

 the living pachyderm order, we shall find that the largest 

 species are of the lowest organization. For example, the 

 eleonant, with its short metatarsus, is a low form compar- 

 ed with the horse, in which the heel is raised so much 

 above the ground. This is a progress of characters which 

 could be shown in many other families. It is a progress 

 which may be generally described as passing from the 

 phocal form of the hind extremities, through the planti- 

 grade and ascending to its ultimatum in the digitigrade. 

 Now this progress is coincident with the distribution of 

 the various lines of animals in physical geography, for 

 while the first are marine the second are generally found 

 in connection with shores, rivers, and low grounds, and 

 the last (always the smallest) with the more varied sur- 

 face of the interior. When we find, then, animals of the 

 second kind most conspicuous in this period, we have ac- 

 tual phenomenon remarkably in accordance with the 

 scheme of development. We look in, as it were, upon the 

 world, or at least, its chief zoological province, at the time 

 when the lines had attained to the terrestrial mammal 

 forms fitted for fluviatile and jungle life, and ere from 

 from these had yet sprung the whole of the smaller but 

 more highly organized denizens of nature's common. 



Our critic, having now run over the whole series of 

 fossils, summons Cuvier, Agassiz, and Owen to egress 

 their opinions against the theory of development. The 

 first " again and again affirms that the extinct fossil spe- 

 cies were not produced by any continued natural organic 

 law from other species." His French opponents tried. 



