636 UNITY OF DESIGN. 



may warrant. It should be borne in mind 

 that these facts are few, compared with the 

 entire history of animal developement ; and that 

 the resemblances which have been so ingeniously 

 traced, are partial only, and fall very short of 

 that universality, which alone constitutes the 

 solid basis of a strictly philosophical theory. 

 Whatever may be the apparent similarity be- 

 tween one animal and another, during different 

 periods of their respective developements, there 

 still exist specific differences, establishing be- 

 tween them an impassable barrier of separation, 

 and effectually preventing any conversion of one 

 species into another, however nearly the two 

 may be mutually allied. The essential charac- 

 ters of each species, amidst occasional varieties, 

 remain ever constant and immutable. Although 

 gradations, to a greater or less*extent, may be 

 traced among the races both of plants and 

 animals, yet in no case is the series strictly 

 continuous ; each step, however short, being in 

 reality an abrupt transition from one type of 

 conformation to another. In many instances the 

 interval is considerable ; as for example in the 

 passage from the invertebrate to the vertebrated 

 classes; and indeed in every instance where 

 great changes in the nature and arrangement of 

 the functions take place.* It is in vain to allege 



* See a paper on this subject, by Cuvier, in the Ann. des 

 Sciences Naturelles, xx. 241. 



