er creature, is of interest, not so much on its
own account as for its associations. He sees it not as an individual
but as a link in the scale of organic things, as the bearer of a certain
message of world-history. Thus the radiolarians, insignificant creatures
though they seem, have really taken an extraordinary share in building
up the crust of the earth. The ooze at the bottom of the sea,
which finally becomes metamorphosed into chalk or stone, is but the
aggregation of the shells of dead radiolarians. In the light of such a
role the animalcule takes on a new interest.

But even greater is the interest that attaches to every creature in
regard to the question of its place in the organic scale of evolution.
What are the homologies of this form and that? What its probable
ancestry? What gaps does it bridge? What can it tell us of the story of
animal creation? These and such like are the questions that have been
ceaselessly before Haeckel's mind in all his studies of zoology. Hence
the rich fountain of philosophical knowledge that has welled up from
what otherwise might have been the most barren of laboratory borings.
Thus from a careful investigation of the sponge Haeckel was led to
his famous gastrula theory, according to which the pouchlike
sponge-animalcule--virtually a stomach without members--is the type of
organism on which all high organisms are built, so to speak--that is,
out of which all have evolved.

This gastrula theory, now generally accepted, is one of Haeckel's two
great fundamental contributions to the evolution philosophy with the
history of which his life work is so intimately linked. The other
contribution is the theory, even more famous and now equally undisputed,
that every individual organism, in its em-bryological development,
rehearses in slurred but unmistakable epitome the steps of evolution by
which the ancestors of that individual came into racial being. That is
to say, every mammal, for example, originating in an egg stage, when it
is comparable to a protozoon, passes through successive stages when it
is virtually in succession a gastrula, a fish, and an amphibian before
it attains the mammalian status, because its direct ancestors were in
succession, through the long geological ages, protozoons, gastrulae,
fishes, amphibians before the true mammal was evolved. This theory cast
a flood of light into many dark places of the Darwinian philosophy. It
was propounded in 1866 in Professor Haec