46 OBSERVATIONS ON THE ANIMALS 
same conclusion, there is certainly a high degree of probability that 
the neighbourhood of the North Pole did not then produce that ex- 
tremity of cold which now characterizes it. At all events, the gene- 
ral opinion at the present day is in favour of such a supposition ; and 
we do not feel either inclined or prepared to controvert it. All we 
wish is, to give the reader to understand that it is a supposition, and 
not a matter absolutely decided. 
We have hitherto, while considering the family Mawtilacea, been 
guided in some measure by analogies which connect the fossil with 
the recent genera. Passing on now to the Ammoneata, we are 
obliged to leave behind us these glimmerings of light ; and since 
there are, so far as is known, no living congeners of this family, we 
are forced to bring into operation certain rules founded on experience, 
and depending on two broad and general principles: viz. that no part 
of an animal whatever exists without its use; and that while the ge- 
neral structure is perfectly adapted to the wants of the species of 
which it is significant, so also every portion has a mutual relation 
with all the rest, is in itself quite perfect, and exactly fitted to the 
purpose assigned to it. 
The extent to which this entire dependence on the perfect wisdom 
of the Creator may be safely carried, must be almost marvellous to 
any person not in the habit of studying Natural History, with en- 
larged and general views. But no one can call to mind its value in 
the hands of Cuvier, when applied to the comparative anatomy of 
animals of higher organization, without acknowledging that it became 
with him an instrument for the discovery of truth, whose astonishing 
power was only equalled by the sagacity of him who employed it. It 
remains still to be seen how far the same means of discovery may 
be extended to the lower forms of animal life, and the conclusions 
forced upon us in consequence. We have already shewn that, in 
applying it to the cephalopodous animals, we are descending only the 
first step in the scale of organization from the vertebrata (the object of 
Cuvier’s researches), and thus, in the case before us, we are entitled 
to look with considerable confidence to that dependence of structure 
on organization which certainly becomes less strongly marked as we 
approach the limits separating the animal from the vegetable exist- 
ence. 
Proceeding on these principles, we assume the position of the 
siphuncles in multilocular shells as the distinctive character of two 
great families, because we find this character accompanied by a very 
remarkable change in the coutrivances which indicate strength, and 
?.. 
