120 ESSAY ON CLASSIFICATION. Part I. 



of this Idud among Batracliians^ (see, above, Sect. XII.) ; among Birds,^ the uniformly 

 webbed foot, in all young, exhibits another correspondence between the young 

 of higher orders and the permanent character of the lower ones. In the order 

 of Caruivora, the Seals, the Plantigrades, and the Digitigrades exemplify the same 

 coincidence between higher and higher representatives of the same types, and the 

 embryonic changes through wliich the highest pass successively. 



No more complete evidence can be needed to show that there exists throughout 

 the animal kingdom the closest correspondence between the gradation of their types 

 and the embryonic changes their respective representatives exhibit throughout. And 

 yet what genetic relation can there exist between the Pentacrinus of the West 

 Indies and the ComatulEe, found in every sea; what between the embryos of Spatan- 

 goids and those of Echinoids, and between the former and the adult Echinus; 

 what between the larva of a Crab and our Lobsters; what between the Caterpillar 

 of a Papilio and an adult Tinea, or an adult Sphinx; what between the Tadpole 

 of a Toad and our Menobranchus ; what between a young Dog and our Seals, 

 unless it be the plan designed by an intelligent Creator? 



SECTION XXVIII. 



RELATIONS BETWEEN THE STRUCTURE, EMBRYONIC GROWTH, GEOLOGICAL SUCCESSION, AND 

 THE GEOGRAPHICAL DISTRIBUTION OF ANIMALS. 



It requires unusual comprehensiveness of view to j^erceive the order jjrevailing 

 in the geographical distribution of animals. We should, therefore, not wonder that 

 this branch of Zoology is so far behind the other divisions of that science. Nor 

 should we wonder at the fact that the geogi'aphical distribution of plants is so much 

 better known than that of animals, when we consider how marked a feature the 

 vegetable carpet which covers the surface of our globe is, when compared with the 

 little show animals make, almost everywhere. And yet it will, perhaps, some da}^, 

 be easier to understand the relations existing between the geographical distribution 

 of animals and the other general relations prevailing among animals, because the 

 range of structural differences is much greater among animals than among plants. 

 Even now, some curious coincidences may be pointed out which go far to show 

 that the geographical distribution of animals stands in direct relation to their rela- 



* Twelve Lectures, etc., p. 8. ' Agassiz, (L.,) Lake Superior, etc., p. 194. 



