their
peculiarities, which are monsters thus produced and propagated. And to
these must be added, the changes produced by the imagination of the male
parent, as will be treated of more at large in No. VI. of this Section.

When we consider all these changes of animal form, and innumerable others,
which may be collected from the books of natural history; we cannot but be
convinced, that the fetus or embryon is formed by apposition of new parts,
and not by the distention of a primordial nest of germs, included one
within another, like the cups of a conjurer.

Fourthly, when we revolve in our minds the great similarity of structure,
which obtains in all the warm-blooded animals, as well quadrupeds, birds,
and amphibious animals, as in mankind; from the mouse and bat to the
elephant and whale; one is led to conclude, that they have alike been
produced from a similar living filament. In some this filament in its
advance to maturity has acquired hands and fingers, with a fine sense of
touch, as in mankind. In others it has acquired claws or talons, as in
tygers and eagles. In others, toes with an intervening web, or membrane, as
in seals and geese. In others it has acquired cloven hoofs, as in cows and
swine; and whole hoofs in others, as in the horse. While in the bird kind
this original living filament has put forth wings instead of arms or legs,
and feathers instead of hair. In some it has protruded horns on the
forehead instead of teeth in the fore part of the upper jaw; in others
tushes instead of horns; and in others beaks instead of either. And all
this exactly as is daily seen in the transmutations of the tadpole, which
acquires legs and lungs, when he wants them; and loses his tail, when it is
no longer of service to him.

Fifthly, from their first rudiment, or primordium, to the termination of
their lives, all animals undergo perpetual transformations; which are in
part produced by their own exertions in consequence of their desires and
aversions, of their pleasures and their pains, or of irritations, or of
associations; and many of these acquired forms or propensities are
transmitted to their posterity. See Sect. XXXI. 1.

As air and water are supplied to animals in sufficient profusion, the three
great objects of desire, which have changed the forms of many animals by
their exertions to gratify them, are those of lust, hunger, and security. A
great want of one part of the animal world has consisted in the desire