individuals
forming the original type, we have a complex mass of unlike parts
fulfilling unlike duties. As an individual _Hydra_ may be regarded as a
group of _Protozoa_ which have become partially metamorphosed into
different organs; so a _Physalia_ is, morphologically considered, a
group of _Hydrae_ of which the individuals have been variously
transformed to fit them for various functions.

This differentiation upon differentiation is just what takes place
during the evolution of a civilized society. We observed how, in the
small communities first formed, there arises a simple political
organization: there is a partial separation of classes having different
duties. And now we have to observe how, in a nation formed by the fusion
of such small communities, the several sections, at first alike in
structures and modes of activity, grow unlike in both--gradually become
mutually-dependent parts, diverse in their natures and functions.

* * * * *

The doctrine of the progressive division of labour, to which we are here
introduced, is familiar to all readers. And further, the analogy between
the economical division of labour and the "physiological division of
labour," is so striking as long since to have drawn the attention of
scientific naturalists: so striking, indeed, that the expression
"physiological division of labour," has been suggested by it. It is not
needful, therefore, to treat this part of the subject in great detail.
We shall content ourselves with noting a few general and significant
facts, not manifest on a first inspection.

Throughout the whole animal kingdom, from the _Coelenterata_ upwards,
the first stage of evolution is the same. Equally in the germ of a
polype and in the human ovum, the aggregated mass of cells out of which
the creature is to arise, gives origin to a peripheral layer of cells,
slightly differing from the rest which they include; and this layer
subsequently divides into two--the inner, lying in contact with the
included yelk, being called the mucous layer, and the outer, exposed to
surrounding agencies, being called the serous layer: or, in the terms
used by Prof. Huxley, in describing the development of the
_Hydrozoa_--the endoderm and ectoderm. This primary division marks out a
fundamental contrast of parts in the future organism. From the mucous
layer, or endoderm, is developed the apparatus of nutrition; while from
the serous layer, or ectoderm, is