like cells, and ultimately of one-celled animals?

Comparative anatomy has asserted that this is so, as we have already
learned, for it finds that adult animals array themselves at different
levels of a scale beginning at the bottom with the protozoa, continuing on
to the two-layered animals like _Hydra_ and jellyfish and sea-anemones,
and then extending upwards to the region of the more complicated
invertebrates and vertebrates. It was difficult perhaps to believe that
these successive grades of organic structure indicated an order of
evolution, because it seemed impossible that an animal so simple as a
protozoan could produce offspring with the complex organization of a frog
or a cat, even in long ages. But development delivers its evidence
relating to this matter with telling and impressive force. How can we
doubt the possibility of an evolution of higher animals from ancestors as
simple as _Hydra_ and _Amoeba_ when a frog and a cat, like all other
complicated organisms, begin individual existence as single cells, and
pass through gastrula stages? If we deny it, we contradict the evidence of
our senses, for the development is actually accomplished by the
transformation of a single cell into a double-walled sac, and of this into
different and more intricate organic mechanisms. The process _can_ take
place, for it _does_ take place. Not until the investigator becomes
familiar with a wide range of diverse animals and the peculiar qualities
of their similar early stages, can he estimate the tremendous weight of
the facts of comparative embryology. Were the statement iterated and
reiterated on every page and in every paragraph, there would be no undue
emphasis put upon the astounding fact that the apparently impassable gap
between a one-celled animal like _Amoeba_ and a mammal like a cat is
actually compassed during the development of the last-named organisms from
single cells. The occurrence of gill-slits in the embryos of lizards,
birds, and mammals now seems a small thing when compared with the