2 ANIMAL PHYSIOLOGY. 
teristics of living things as such? By what barriers are the 
animate and inanimate worlds separated? To decide this, 
falls within the province of general biology. 
Living things grow by interstitial additions of particles of 
matter derived from without and transformed into their own 
substance, while inanimate bodies increase in size by superfi- 
cial additions of matter over which they have no power of 
decomposition and recomposition so as to make them like 
themselves. Among lifeless objects, crystals approach near- 
est to living forms; but the crystal builds itself up only from 
material in solution of the same chemical composition as itself. 
The chemical constitution of living objects is peculiar. Car- 
bon, hydrogen, oxygen, and nitrogen are combined into a very 
complex whole or molecule, as protein; and, when in com- 
bination with a large proportion of water, constitute the basis 
of all life, animal and vegetable, known as protoplasm. Only 
living things can manufacture this substance, or even protein. 
Again, in the very nature of the case, protoplasm is con- 
tinually wasting by a process of oxidation, and being built up 
from simpler chemical forms. Carbon dioxide is an invariable 
product of this waste and oxidation, while the rest of the car- 
bon, the hydrogen, oxygen, and nitrogen are given back to the 
inorganic kingdom in simpler forms of combination than those 
in which they exist in living beings. It will thus be evident 
that, while the flame of life continues to burn, there is constant 
chemical and physical change. Matter is being continuously 
taken from the world of things that are without life, trans- 
formed into living things, and then after a brief existence in 
that form returned to the source from which they were origi- 
nally derived. It is true, all animals require their food in or- 
ganized form—that is, they either feed on animal or plant 
forms; but the latter derive their nourishment from the soil 
and the atmosphere, so that the above statement is a scientific 
truth. 
Another highly characteristic property of all living things 
is to be sought in their periodic changes and very limited dura- 
tion. Every animal and plant, no matter what its rank in the 
scale of existence, begins in a simple form, passes through a 
series of changes of varying degrees of complexity, and finally 
declines and dies; which simply means that it rejoins the in- 
animate kingdom : it passes into another world to which it 
formerly belonged. 
Living things alone give rise to living things; protoplasm 
