e, there is analogy alike in the slight extent to which
organization is carried, in the indefiniteness of this organization, and
in its want of fixity.

A further complication of the analogy is at hand. From the aggregation
of units into organized groups, we pass to the multiplication of such
groups, and their coalescence into compound groups. The _Hydra_, when it
has reached a certain bulk, puts forth from its surface a bud which,
growing and gradually assuming the form of the parent, finally becomes
detached; and by this process of gemmation the creature peoples the
adjacent water with others like itself. A parallel process is seen in
the multiplication of those lowly-organized tribes above described. When
one of them has increased to a size that is either too great for
co-ordination under so rude a structure, or else that is greater than
the surrounding country can supply with game and other wild food, there
arises a tendency to divide; and as in such communities there often
occur quarrels, jealousies, and other causes of division, there soon
comes an occasion on which a part of the tribe separates under the
leadership of some subordinate chief and migrates. This process being
from time to time repeated, an extensive region is at length occupied by
numerous tribes descended from a common ancestry. The analogy by no
means ends here. Though in the common _Hydra_ the young ones that bud
out from the parent soon become detached and independent; yet throughout
the rest of the class _Hydrozoa_, to which this creature belongs, the
like does not generally happen. The successive individuals thus
developed continue attached; give origin to other such individuals which
also continue attached; and so there results a compound animal. As in
the _Hydra_ itself we find an aggregation of units which, considered
separately, are akin to the lowest _Protozoa_; so here, in a _Zoophyte_,
we find an aggregation of such aggregations. The like is also seen
throughout the extensive family of _Polyzoa_ or _Molluscoida_. The
Ascidian Mollusks, too, in their many forms, show us the same thing:
exhibiting, at the same time, various degrees of union among the
component individuals. For while in the _Salpae_ the component
individuals adhere so slightly that a blow on the vessel of water in
which they are floating will separate them; in the _Botryllidae_ there
exist vascular connexions among them, and a common circulation. Now in
these different sta