of a membrane round each
of the masses of granules into which the endochrome of an alga-cell
breaks up, is an instance of analogous kind. And should the
recently-asserted fact that cells may arise round vacuoles in a mass of
organizable substance, be confirmed, another good example will be
furnished; for such portions of substance as bound these vacant spaces
are subject to influences unlike those to which other portions of the
substance are subject. If then we can most clearly trace this law of
modification in these primordial processes, as well as in those more
complex but analogous ones exhibited in the early changes of an ovum, we
have strong reason for thinking that the law is fundamental.

But, as already more than once hinted, this principle, understood in the
simple form here presented, supplies no key to the detailed phenomena of
organic development. It fails entirely to explain generic and specific
peculiarities; and leaves us equally in the dark respecting those more
important distinctions by which families and orders are marked out. Why
two ova, similarly exposed in the same pool, should become the one a
fish, and the other a reptile, it cannot tell us. That from two
different eggs placed under the same hen, should respectively come forth
a duckling and a chicken, is a fact not to be accounted for on the
hypothesis above developed. Here we are obliged to fall back upon the
unexplained principle of hereditary transmission. The capacity possessed
by an unorganized germ of unfolding into a complex adult which repeats
ancestral traits in minute details, and that even when it has been
placed in conditions unlike those of its ancestors, is a capacity
impossible for us to understand. That a microscopic portion of seemingly
structureless matter should embody an influence of such kind, that the
resulting man will in fifty years after become gouty or insane, is a
truth which would be incredible were it not daily illustrated. But
though the _manner_ in which hereditary likeness, in all its
complications, is conveyed, is a mystery passing comprehension, it is
quite conceivable that it is conveyed in subordination to the law of
adaptation above explained; and we are not without reasons for thinking
that it is so. Various facts show that acquired peculiarities resulting
from the adaptation of constitution to conditions, are transmissible to
offspring. Such acquired peculiarities consist of differences of
structure or comp