like creature, called by naturalists the _Echinococcus_--a
creature so extremely different from the tape-worm in aspect and
structure, that only after careful investigations has it been proved to
have the same origin. All which instances imply that each advance in
embryonic complication results from the action of incident forces upon
the complication previously existing.

Indeed, we may find _a priori_ reason to think that the evolution
proceeds after this manner. For since it is now known that no germ,
animal or vegetable, contains the slightest rudiment, trace, or
indication of the future organism--now that the microscope has shown us
that the first process set up in every fertilised germ, is a process of
repeated spontaneous fissions ending in the production of a mass of
cells, not one of which exhibits any special character: there seems no
alternative but to suppose that the partial organisation at any moment
subsisting in a growing embryo, is transformed by the agencies acting
upon it into the succeeding phase of organisation, and this into the
next, until, through ever-increasing complexities, the ultimate form is
reached. Thus, though the subtilty of the forces and the slowness of the
results, prevent us from _directly_ showing that the stages of
increasing heterogeneity through which every embryo passes, severally
arise from the production of many changes by one force, yet,
_indirectly_, we have strong evidence that they do so.

We have marked how multitudinous are the effects which one cause may
generate in an adult organism; that a like multiplication of effects
must happen in the unfolding organism, we have observed in sundry
illustrative cases; further, it has been pointed out that the ability
which like germs have to originate unlike forms, implies that the
successive transformations result from the new changes superinduced on
previous changes; and we have seen that structureless as every germ
originally is, the development of an organism out of it is otherwise
incomprehensible. Not indeed that we can thus really explain the
production of any plant or animal. We are still in the dark respecting
those mysterious properties in virtue of which the germ, when subject to
fit influences, undergoes the special changes that begin the series of
transformations. All we aim to show, is, that given a germ possessing
these mysterious properties, the evolution of an organism from it,
probably depends upon that multiplic