ALL THE ANIMALS, INCLUDING MAN, DESCEND FROM A UNICELLULAR ORGANISM.
If our biogenetic law is true, if the embryonic development is a
summary or condensed recapitulation of the stem-history--and there can
be no doubt about it--we are bound to conclude, from the fact that all
the ova are at first simple cells, that all the multicellular
organisms originally sprang from a unicellular being. And as the
original ovum in man and all the other animals has the same simple and
indefinite appearance, we may assume with some probability that this
unicellular stem-form was the common ancestor of the whole animal
world, including man. However, this last hypothesis does not seem to
me as inevitable and as absolutely certain as our first conclusion.

This inference from the unicellular embryonic form to the unicellular
ancestor is so simple, but so important, that we cannot sufficiently
emphasise it. We must, therefore, turn next to the question whether
there are to-day any unicellular organisms, from the features of which
we may draw some approximate conclusion as to the unicellular
ancestors of the multicellular organisms. The answer is: Most
certainly there are. There are assuredly still unicellular organisms
which are, in their whole nature, really nothing more than permanent
ova. There are independent unicellular organisms of the simplest
character which develop no further, but reproduce themselves as such,
without any further growth. We know to-day of a great number of these
little beings, such as the gregarinae, flagellata, acineta, infusoria,
etc. However, there is one of them that has an especial interest for
us, because it at once suggests itself when we raise our question, and
it must be regarded as the unicellular being that approaches nearest
to the real ancestral form. This organism is the amoeba.

For a long time now we have comprised under the general name of