96 EXPLANATIONS. 



form such a conclusion. A " confusion of genera 

 or species" is not to be presumed ; there is no 

 need for a shading of structure to make dim thei 

 outlines. I suggest, that a line of organization, 

 analogous to the progress of the embryo of aq 

 elevated species, had passed in the course of tirai 

 through its appointed stages of development, eacl 

 of which is a small advance upon the preceding, 

 and the type of a form thenceforth to continue per? 

 manent. Each line stands apart. It may show 

 shadings in a vertical direction, as between ita 

 reptilian and its mammal forms, but no true affini- 

 ties connecting horizontally with the members ol 

 other lines. Our critic is here, therefore, com- 

 pletely at fault. I meet him again, however, oi]k 

 special grounds. Many of the animals of the ter- 

 tiary 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 creatures equally vast in proportion belong-" 

 ing to the rodent, the edentate, and other orders. 

 These huge mammals are, indeed, the signal forms 

 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. 



