corps vivants de la nature," into which Payen had, so early, a clear
insight.

As far back as 1850, Cohn wrote, apparently without any knowledge of
what Payen had said before him:--

"The protoplasm of the botanist, and the contractile substance
and sarcode of the zoologist, must be, if not identical, yet
in a high degree analogous substances. Hence, from this point
of view, the difference between animals and plants consists
in this; that, in the latter, the contractile substance, as
a primordial utricle, is enclosed within an inert cellulose
membrane, which permits it only to exhibit an internal motion,
expressed by the phenomena of rotation and circulation, while,
in the former, it is not so enclosed. The protoplasm in the
form of the primordial utricle is, as it were, the animal
element in the plant, but which is imprisoned, and only
becomes free in the animal; _or_, to strip off the metaphor
which obscures simple thought, the energy of organic vitality
which is manifested in movement is especially exhibited by a
nitrogenous contractile substance, which in plants is limited
and fettered by an inert membrane, in animals not so."[1]

[Footnote 1: Cohn, "Ueber Protococcus pluvialis," in the "Nova Acta"
for 1850.]

In 1868, thinking that an untechnical statement of the views current
among the leaders of biological science might be interesting to the
general public, I gave a lecture embodying them in Edinburgh. Those
who have not made the mistake of attempting to approach biology,