Notes. 
605 
they are, are not themselves protoplasm. Their cumbrous molecules 
are built up and broken down by ordinary chemical processes. They 
are not in themselves, in any intelligible sense, living, though essential 
to the exhibition of vital phenomena. 
There our analysis of living matter by physical methods for the 
present stops. But we are justified in pushing, at any rate', semi- 
physical conceptions as far as we dare. We conceive, therefore, the 
physical constituent molecules of protoplasm as aggregated into larger 
molecules which, as they are unlike anything we know as purely 
physical, we call physiological l . Of the properties of such molecules 
we have some faint conceptions. The first is their instability. They 
are kinetic ; ‘ living substance is continually breaking down into 
simpler bodies, with a setting free of energy; on the other hand 
living substance is continually building itself up, embodying energy 
into itself, and so replenishing its store of energy 2 / This kinetic 
condition is essentially life ; when it ceases, we have hitherto believed 
that the constituents of protoplasm come under the sway of purely 
inorganic conditions. 
If we pause for a moment to attempt a quasi-mechanical explana- 
tion of the more developed phenomena of living organisms, such for 
example as are included under heredity, we are led to suppose that 
the physiological molecules may themselves be grouped into larger 
aggregates. And each stage of aggregation introduces us into a new 
order of phenomena. All that we can say is, that beyond the first 
stage the properties which are characteristic of higher molecular 
aggregrates are ultra-physical, taking physical in its ordinary 
signification. That does not imply, however, that physical conditions 
are ever in abeyance. Each stage of aggregation is conditioned 
by every one that precedes it. In this sense life rests au fond on 
a physical basis. 
A continuous kinetic condition appears to be one distinctive pro- 
perty of physiological molecules. This not merely manifests itself 
in continuous chemical activity, but under appropriate conditions in 
actual visible motion. And it is to be remarked of the former, that 
though chemical in kind, it is undoubtedly ultra-chemical, as chemistry 
is understood in the laboratory. A further characteristic of the 
1 Identical with Foster’s * somacula/ Textbook of Physiology, Part I, fifth ed., 
p. 6. 
2 Foster, loc. cit., p. 41. 
