230 STUDY OF NATURAL HISTORY. 



particular parts of the creation, which have been 

 carefully examined, it may hold good in all. True 

 it is, that nature does not always proceed pari passu. 

 In the Linnaean genus Psittacus, — a group of very 

 limited structure, — the chain is composed of a great 

 number of links ; whereas in Pachydermata, — a group 

 presenting a very wide range of structure, — the 

 number of links is comparatively small. Still there 

 is continuity manifest in both; the difference de- 

 pending merely on the relative distance between 

 some two contiguous forms in each. Chasms in the 

 chain may be numerous and small, as in Psittacus ; 

 or few and wide, as in Pachydermata. — Continuity 

 in gradation of structure cannot exist, as we have 

 seen, without intervals; and the size of these intervals 

 does not lessen the truth of the chain, because some 

 of the links may not yet be discovered. How, then, 

 it may be asked, are we to prove that the chain is 

 continuous ? The reply is, simply by ascertaining 

 which animals of one group come the nearest to 

 those of the other. If there be no approximation — 

 if all the animals remain equally distant — then there 

 is no continuity; but if one animal of the one group 

 approaches to the structure of the other, then there 

 is a chain of continuity — possessing, indeed, but 

 only one link', but not the less presenting a mode of 

 transition from one form to the other. Thus, if the 

 only animal existing between quadrupeds and fishes 

 were one penguin, it would still be in the path of 

 passage. But if a tortoise existed in addition, the 

 chain would be more complete ; and if one frog 

 existed also, the chain would scarcely escape notice. 

 In it, there is a regular and obvious gradation of 



