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 Linnean 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 Psettacus ; 
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 
