The Evolution of Man, vol 2 | Page 4

heredity--whole
series of lower stages have dropped out in the embryonic development
of man and the other mammals especially from the earliest periods, or
been falsified by modification. But we find these lower stages in their
original purity in the lower vertebrates and their invertebrate ancestors.
Especially in the lowest of all the vertebrates, the lancelet or
Amphioxus, we have the oldest stem-forms completely preserved in the
embryonic development. We also find important evidence in the fishes,
which stand between the lower and higher vertebrates, and throw
further light on the course of evolution in certain periods. Next to the
fishes come the amphibia, from the embryology of which we can also
draw instructive conclusions. They represent the transition to the higher
vertebrates, in which the middle and older stages of ancestral
development have been either distorted or curtailed, but in which we
find the more recent stages of the phylogenetic process well preserved
in ontogeny. We are thus in a position to form a fairly complete idea of
the past development of man's ancestors within the vertebrate stem by
putting together and comparing the embryological developments of the

various groups of vertebrates. And when we go below the lowest
vertebrates and compare their embryology with that of their
invertebrate relatives, we can follow the genealogical tree of our animal
ancestors much farther, down to the very lowest groups of animals.
In entering the obscure paths of this phylogenetic labyrinth, clinging to
the Ariadne-thread of the biogenetic law and guided by the light of
comparative anatomy, we will first, in accordance with the methods we
have adopted, discover and arrange those fragments from the manifold
embryonic developments of very different animals from which the
stem-history of man can be composed. I would call attention
particularly to the fact that we can employ this method with the same
confidence and right as the geologist. No geologist has ever had ocular
proof that the vast rocks that compose our Carboniferous or Jurassic or
Cretaceous strata were really deposited in water. Yet no one doubts the
fact. Further, no geologist has ever learned by direct observation that
these various sedimentary formations were deposited in a certain order;
yet all are agreed as to this order. This is because the nature and origin
of these rocks cannot be rationally understood unless we assume that
they were so deposited. These hypotheses are universally received as
safe and indispensable "geological theories," because they alone give a
rational explanation of the strata.
Our evolutionary hypotheses can claim the same value, for the same
reasons. In formulating them we are acting on the same inductive and
deductive methods, and with almost equal confidence, as the geologist.
We hold them to be correct, and claim the status of "biological
theories" for them, because we cannot understand the nature and origin
of man and the other organisms without them, and because they alone
satisfy our demand for a knowledge of causes. And just as the
geological hypotheses that were ridiculed as dreams at the beginning of
the nineteenth century are now universally admitted, so our
phylogenetic hypotheses, which are still regarded as fantastic in certain
quarters, will sooner or later be generally received. It is true that, as
will soon appear, our task is not so simple as that of the geologist. It is
just as much more difficult and complex as man's organisation is more
elaborate than the structure of the rocks.
When we approach this task, we find an auxiliary of the utmost
importance in the comparative anatomy and embryology of two lower

animal-forms. One of these animals is the lancelet (Amphioxus), the
other the sea-squirt (Ascidia). Both of these animals are very instructive.
Both are at the border between the two chief divisions of the animal
kingdom--the vertebrates and invertebrates. The vertebrates comprise
the already mentioned classes, from the Amphioxus to man (acrania,
lampreys, fishes, dipneusts, amphibia, reptiles, birds, and mammals).
Following the example of Lamarck, it is usual to put all the other
animals together under the head of invertebrates. But, as I have often
mentioned already, the group is composed of a number of very
different stems. Of these we have no interest just now in the
echinoderms, molluscs, and articulates, as they are independent
branches of the animal-tree, and have nothing to do with the vertebrates.
On the other hand, we are greatly concerned with a very interesting
group that has only recently been carefully studied, and that has a most
important relation to the ancestral tree of the vertebrates. This is the
stem of the Tunicates. One member of this group, the sea-squirt, very
closely approaches the lowest vertebrate, the Amphioxus, in its
essential internal structure and embryonic development. Until 1866 no
one had any idea of the close connection of these apparently very
different