ORIGIN OF LIFE. 71 
between the sum total of molecular actions taking 
place within the living aggregate and the forces of 
its environment. 
The power of undergoing spontaneous division 
(fission or gemmation), which is manifested by living 
matter, and upon which all the phenomena of ‘re- 
production’ depend, is apparently one of its most 
fundamental properties, though it is in itself a result 
of that molecular mobility and complexity to which 
we have previously referred. 
And it is this same molecular mobility which 
makes an aggregate of living matter, in the form of 
a simple organism, very prone to undergo changes 
in its intimate constitution—either ‘spontaneously,’ 
or under the incidence of a known change in external 
forces. Some new conditions may not visibly affect 
it, others may cause its ‘death,’ whilst others again 
may affect it only to such an extent as to bring 
about some modification of its molecular constitution, 
which, by reason of an altered polarity, entails a 
more or less marked transformation of form and 
structure (Heterogenesis). 
Thus the marvellous convertibility of lower organ- 
isms, their ability to undergo self-multiplication, 
and their tendency to become (under favourable con- 
ditions) more complexly organized, are all necessary 
consequences of those physical doctrines concerning 
