protoplasm are liable to undergo that peculiar coagulation at a
temperature of 40°-50° Centigrade, which has been called
"heat-stiffening," though Kühne's beautiful researches have proved this
occurrence to take place in so many and such diverse living beings, that
it is hardly rash to expect that the law holds good for all.

Enough has, perhaps, been said, to prove the existence of a general
uniformity in the character of the protoplasm, or physical basis, of
life, in whatever group of living beings it may be studied. But it will
be understood that this general uniformity by no means excludes any
amount of special modifications of the fundamental substance. The
mineral, carbonate of lime, assumes an immense diversity of characters,
though no one doubts that, under all these Protean changes, is one and
the same thing.

And now, what is the ultimate fate, and what the origin, of the matter
of life?

Is it, as some of the older naturalists supposed, diffused throughout
the universe in molecules, which are indestructible and unchangeable in
themselves; but, in endless transmigration, unite in innumerable
permutations, into the diversified forms of life we know? Or, is the
matter of life composed of ordinary matter, differing from it only in
the manner in which its atoms are aggregated? Is it built up of ordinary
matter, and again resolved into ordinary matter when its work is done?

Modern science does not hesitate a moment between these alternatives.
Physiology writes over the portals of life--

"Debemur morti nos nostraque,"[51]