‘Prof. LeCoute on the Correlation of Forces. 145 
out of the system? The answer usually given is that such food 
is used in the animal economy solely as fuel to keep up the ani- 
mal heat. On this view it is difficult to see why this class of 
food should be used at all, especially in warm climates. But ac- 
cording to the view which I propose we have here an additional 
source of vital force. The decomposition of these ternary com- 
pounds sets free a force which is used in organizing and assimi- 
lating other matter (nitrogenous) and in producing animal acti- 
vity and animal heat- As in plants, although the decomposition 
of CO? by sunlight is all that is absolutely necessary for growth 
and development, yet the decomposition of organic food sup- 
plies an additional force which greatly increases the vigour and 
rapidity of vegetation; so in animals, although decomposition 
of the tissues is all that is absolutely necessary to furnish the 
force of growth and the phenomena of animal life generally, yet 
the decomposition of non-nitrogenous organic food furnishes ad- 
ditional force by which growth and animal activity may be 
maintained without too great an expenditure of the tissues. 
11th. In what then consists the essential difference between 
animals and plants? ‘There can be no doubt that it consists, 
generally, in their relations to one another and to the mineral 
kingdom. Plants occupy a middle ground between the mineral 
and animal kingdom—a necessary halting-place for matter in its 
upward struggles. But when we attempt to define this relation 
more accurately, the problem becomes much more difficult. It 
is indeed probable that no single distinction will be found free 
from objection. The commonly received and, to a certain ex- 
tent, very correct idea, is that the essential distinction consists 
in their relation to CO?. Plants decompose, and animals recom- 
ose CO?. The beautiful manner in which the two kingdoms 
stand related to each other through these converse processes is 
familiar to all. But it is well known that most plants carry on 
both of these processes at the same time; while some, as fungi, 
pale plants, &c., only reeompose CO?, like animals. It seems to 
me that at least an equally good fundamental distinction may 
be found in this: that in plants the fundamental and necessary 
source of vital force is the decomposition of its mineral food ; 
while in animals the fundamental source of vital force is the de- 
composition of its tissues. It is true that in what I have called 
the supplementary source of vital force they seem to meet on 
common ground, viz. the decomposition of organic food ; but 
even here there is this essential difference, that in plants this de- 
composition of organic food is only partial, and therefore fur- 
nishes not only force but material for organization ; while in 
animals the decomposition is complete and therefore furnishes 
only force. 
As a necessary result of the above, it would seem that the 
Phil. Mag. 8. 4, Vol. 19. No, 125, Feb. 1860. 
