ce of new developments;
for, if the animal is deprived of these developments, those changes do
not take place. These changes I believe to be formed not by elongation
or distension of primeval stamina, but by apposition of parts; as the
mature crab fish when deprived of a limb, in a certain space of time,
has power to regenerate it; and the tadpole puts forth its feet after
its long exclusion from the spawn, and the caterpillar in changing into
a butterfly acquires a new form with new powers, new sensations, and new
desires."[174]

. . . . . .

"From hence I conclude that with the acquisition of new parts, new
sensations and new desires, as well as new powers are produced; and this
by accretion to the old ones and not by distension of them. And finally,
that the most essential parts of the system, as the brain for the
purpose of distributing the powers of life, and the placenta for the
purpose of oxygenating the blood, and the additional absorbent vessels,
for the purpose of acquiring aliment, are first formed by the
irritations above mentioned, and by the pleasurable sensations attending
those irritations, and by the exertions in consequence of painful
sensations similar to those of hunger and suffocation. After these an
apparatus of limbs for future uses, or for the purpose of moving the
body in its present natant state, and of lungs for future respiration,
and of _testes_ for future reproduction, are formed by the irritations
and sensations and consequent exertions of the parts previously
existing, and to which the new parts are to be attached.[175]

. . . . . .

"The embryon" must "be supposed to be a living filament, which acquires
or makes new parts, with new irritabilities as it advances in its
growth."[176]

. . . . . .

"From this account of reproduction it appears that all animals have a
similar origin, viz. a single living filament; and that the difference
of their forms and qualities has arisen only from the different
irritabilities and sensibilities, or voluntarities, or associabilities,
of this original living filament, and perhaps in some degree from the
different forms of the particles of the fluids by which it has at first
been stimulated into activity."[177]

. . . . . .

"All animals, therefore, I contend, have a similar cause of their
organization, originating from a single living filament, endued with
different kinds of irritabilities and sensibilities, or of animal
ap