576 
communication with the environment is still the sensitive 
nerve which responds to the stimulus of light,—far more 
sensitive probably than the undifferentiated primordial 
protoplasm. 
In dealing with the phenomenon of fission, Mr. Spencer 
affirms that we are utterly in the dark respecting the 
causes which set up that process everywhere occurring 
among the minutest forms of life ; but he goes on to say 
that “when established, it furthered the spread of those 
which were most favourably differentiated by the 
medium.”* 
But, in the first place, it may be asked whether we are 
altogether in the dark as to the causes which led to the 
fission of an amoeboid substance. 
Mr. J. Arthur Thompson says :— 
“ When the limit of growth is reached, the cell divides. The 
necessity for this division has been partly explained by Spencer and 
Leuckart. If you take a round lump of dough, weighing an ounce, 
another of two ounces, a third of four ounces, you obviously have 
three masses successively doubled, but in doubling the mass you have 
not doubled the surface. The mass increases as the cube, the surface 
only as the square, of the radius. Suppose these lumps alive, the 
second has twice as much living matter as the first, but not twice the 
surface. Yet it is through the surface that the living matter is fed, 
aerated and purified. The unit will therefore get into physiological 
difficulties as it grows bigger, because its increase of surface does not 
keep pace with its increase of mass. Its waste tends to exceed its 
repair, its expenditure gains on its income. What are the alter 
natives? It may go on growing and die (but this is not likely), it 
may cease growing at the fit limit, it may greatly increase its surface 
by outflowing processes (which thus may be regarded as life-saving), 
or it may divide. The last is the usual course. When the unit has 
grown as large as it can conveniently grow, it divides ; in other 
words, it reproduces at the limit of growth, when processes of waste 
are gaining on those of construction. By dividing, the mass is 
lessened, the surface increased, the life continued.”—(The Study of 
Animal Life. p. 182.) 
A 73'