m and its growth are dependent on the reception of
certain principles identical with the chief constituents of blood.

In this sense we may say that the animal organism gives to the blood
only its form; that it is incapable of creating blood out of other
substances which do not already contain the chief constituents of
that fluid. We cannot, indeed, maintain that the animal organism has
no power to form other compounds, for we know that it is capable of
producing an extensive series of compounds, differing in composition
from the chief constituents of blood; but these last, which form the
starting-point of the series, it cannot produce.

The animal organism is a higher kind of vegetable, the development
of which begins with those substances with the production of which
the life of an ordinary vegetable ends. As soon as the latter has
borne seed, it dies, or a period of its life comes to a termination.

In that endless series of compounds, which begins with carbonic
acid, ammonia, and water, the sources of the nutrition of
vegetables, and includes the most complex constituents of the animal
brain, there is no blank, no interruption. The first substance
capable of affording nutriment to animals is the last product of the
creative energy of vegetables.

The substance of cellular tissue and of membranes, of the brain and
nerves, these the vegetable cannot produce.

The seemingly miraculous in the productive agency of vegetables
disappears in a great degree, when we reflect that the production of
the constituents of blood cannot appear more surprising than the
occurrence of the fat of beef and mutton in cocoa beans, of human
fat in olive-oil, of the principal ingredient of butter in palm-oil,
and of horse fat and train-oil in certain oily seeds.

LETTER IX

My dear Sir,

The facts detailed in my last letter will satisfy you as to the
manner in which the increase of mass in an animal, that is, its
growth, is accomplished; we have still to consider a most important
question, namely, the function performed in the animal system by
substances destitute of nitrogen; such as sugar, starch, gum,
pectine, &c.

The most extensive class of animals, the graminivora, cannot live
without these substances; their food must contain a certain amount
of one or more of them, and if these compounds are not supplied,
death quickly ensues.

This important inquiry extends also to the constituents of the food
of carnivorous ani