ssively to the condition of the brain of the three inferior
classes.

"Nature often presents us with this last phenomenon in monsters, but
never exhibits the first. Among the various deformities which organized
beings may experience, they never pass the limits of their own classes
to put on the forms of the class above them. Never does a fish elevate
itself so as to assume the form of the brain of a reptile; nor does the
latter ever attain that of birds; nor the bird that of the mammifer. It
may happen that a monster may have two heads; but the conformation of
the brain always remains circumscribed narrowly within the limits of its
class."[840]

Dr. Clark of Cambridge, in a memoir on "Foetal Development" (1845),
has shown that the concurrent labours of Valentin, Ratka, and Bischoff
disprove the reality of the supposed anatomical analogy between the
embryo condition of certain organs in the higher orders, and the perfect
structure of the same organs in animals of an inferior class. The hearts
and brains, for example, of birds and mammals do not pass through forms
which are permanent in fishes and reptiles; there is only just so much
resemblance as may point to a unity of plan running through the
organization of the whole series of vertebrated animals; but which lends
no support whatever to the notion of a gradual transmutation of one
species into another; least of all of the passage, in the course of many
generations, from an animal of a more simple to one of a more complex
structure.

_Recapitulation._--For the reasons, therefore, detailed in this and the
two preceding chapters, we may draw the following inferences in regard
to the reality of _species_ in nature:--

1st. That there is a capacity in all species to accommodate themselves,
to a certain extent, to a change of external circumstances, this extent
varying greatly, according to the species.

2ndly. When the change of situation which they can endure is great, it
is usually attended by some modifications of the form, colour, size,
structure, or other particulars; but the mutations thus superinduced are
governed by constant laws, and the capability of so varying, forms part
of the permanent specific character.

3dly. Some acquired peculiarities, of form, structure, and instinct, are
transmissible to the offspring; but these consist of such qualities and
attributes only as are intimately related to the natural wants and
propensities of the species.

4thly.