UNITY OF DESIGN. 560 



interpretation of the facts themselves may warrant. 

 It should be borne in mind that these facts are few, 

 compared with the entire history of animal deve- 

 lopement ; 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 

 between one animal and another, during different 

 periods of their respective developements, there 

 still exist specific differences, establishing between 

 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 characters 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 ex- 

 ample in the passage from the invertebrate to the 

 vertebrated classes ; and indeed in every instance 

 where great changes in the nature and arrangement 

 of thg functions take place.* It is in vain to allege 

 that the original continuity of the series is indicated 

 by a few species presenting, in some respects, 

 intermediate characters, sucli as the Ornilliorlnju- 

 chus, between birds and mammalia, and the Cclacea, 



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

 Naturelles, xx. 241 : and also his remarks contained in tlic second 

 edition of his Lemons d'Anat. Comparce. i. ()1, lOU, 1(30, &c. 



