igested and
assimilated. Nevertheless, this complex animal multiplies by division, as
the monad does, and, like the monad, undergoes conjugation. It stands in
the same relation to _Heteromita_ on the animal side, as _Coleochaete_
does on the plant side. Start from either, and such an insensible series
of gradations leads to the monad that it is impossible to say at any
stage of the progress where the line between the animal and the plant
must be drawn.

There is reason to think that certain organisms which pass through a
monad stage of existence, such as the _Myxomycetes_, are, at one time of
their lives, dependent upon external sources for their protein matter, or
are animals; and, at another period, manufacture it, or are plants. And
seeing that the whole progress of modern investigation is in favour of
the doctrine of continuity, it is a fair and probable speculation--though
only a speculation--that, as there are some plants which can manufacture
protein out of such apparently intractable mineral matters as carbonic
acid, water, nitrate of ammonia, metallic and earthy salts; while others
need to be supplied with their carbon and nitrogen in the somewhat less
raw form of tartrate of ammonia and allied compounds; so there may be yet
others, as is possibly the case with the true parasitic plants, which can
only manage to put together materials still better prepared--still more
nearly approximated to protein--until we arrive at such organisms as the
_Psorospermioe_ and the _Panhistophyton_, which are as much animal as
vegetable in structure, but are animal in their dependence on other
organisms for their food.

The singular circumstance observed by Meyer, that the _Torula_ of yeast,
though an indubitable plant, still flourishes most vigorously when
supplied with the complex nitrogenous substance, pepsin; the probability
that the _Peronospora_ is nourished directly by the protoplasm of the
potato-plant; and the wonderful facts which have recently been brought to
light respecting insectivorous plants, all favour this view; and tend to
the conclusion that the difference between animal and plant is one of
degree rather than of kind, and that the problem whether, in a given
case, an organism is an animal or a plant, may be essentially insoluble.

VII

A LOBSTER; OR, THE STUDY OF ZOOLOGY

[1861]

Natural history is the name familiarly applied to the study of the
properties of such natural bodies as minerals, plant