ctural relations established between
these groups by anatomical features in the adult that we must decide
this question. We must examine it also from the embryological point of
view. Every animal in its growth undergoes a succession of changes: is
there anything in these changes implying a transition of one type into
another? Baer has given us the answer to this question. He has shown
that there are four distinct modes of development, as well as four plans
of structure; and though we have seen that higher animals of one class
pass through phases of growth in which they transiently resemble lower
animals of the same class, yet each one of these four modes of
development is confined within the limits of the type, and a Vertebrate
never resembles, at any stage of its growth, anything but a Vertebrate,
or an Articulate anything but an Articulate, or a Mollusk anything but a
Mollusk, or a Radiate anything but a Radiate.

Yet, although there is no embryological transition of one type into
another, the gradations of growth within the limits of the same type and
the same class, already alluded to, are very striking throughout the
Animal Kingdom. There are periods in the development of the germs of the
higher members of all the types, when they transiently resemble in their
general outline the lower representatives of the same type, just as we
have seen that the higher orders of one class pass through stages of
development in which they transiently resemble lower orders of the same
class. This gradation of growth corresponds to the gradation of rank in
adult animals, as established upon comparative complication of
structure. For instance, according to their structural character, all
naturalists have placed Fishes lowest in the scale of Vertebrates. Now
all the higher Vertebrates have a Fish-like character at first, and pass
successively through phases in which they vaguely resemble other lower
forms of the same type before they assume their own characteristic form;
and this is equally true of the other great divisions, so that the
history of the individual is, in some sort, the history of its type.

There is still another aspect of this question,--that of time. If
neither the gradation of structural rank among adult animals, nor the
gradation of growth in their embryological development gives us any
evidence of a transition between types, does not the sequence of animals
in their successive introduction upon the globe afford any