BY MOLECULAR COALESCENCE, 95 
tended for the very purpose, it seems to be unavoidable— 
then vitality must prevent them from acting chemically 
on each other; the vperation of their chemical affinities 
must be overruled, and rendered inoperative. And why 
ali this opposition? Simply that vitality may be afforded 
an opportunity of doing that which it has been demon- 
strated can be just as well done without its aid or inter- 
ference! As this is the only conclusion which such a 
supposition leads to, there is, I think, overwhelming evi- 
dence that the formation of the globules of carbonate of 
lime—the organic-cells of some authors—in the shells of 
crustaceans is due directly to physical and chemical 
agency, and that these globules are identical in structure 
and in their mode of formation with the artificial globules. 
But, lest it should be inferred from all that has been 
stated that I regard the formation of the shells of crusta- 
ceans as entirely physical, a few words of explanation, as 
to the limit which may be assigned to the operation of 
this agency in the formation of these structures, are neces- 
sary. Without pretending to draw an exact line of de- 
marcation between what is physical and what physiologi- 
cal, I may observe that there is just one fact deserving 
notice which goes a great way towards effecting this, 
which is, that when vitality makes use of material sub- 
stances it never disunites them from those influences 
which are at all times acting upon them, but, on the con- 
trary, it employs these influences, making them subserve 
its own purposes, whether the matter employed be in the 
simple condition of a crystal or the complex form of a 
vegetable or an animal structure. Hence, in the formation 
of the shells of crustaceans, vitality devises the means by 
which the materials composing them are brought together, 
adjusts their proportions, and regulates the conditions 
