84
The Presidential Address.
ontogeny, as Haeckel expresses it, is the racial development
or phylogeny in nuce At a certain stage of its development
the human embryo, for example, has gill-slits and branching
arteries as in fishes, and at later stages it is tailed and hairy.
What do such facts mean, if they are not revelations that
man's ancestors have passed through an aquatic stage, and
later through that of a tailed hairy mammal ? From the
Darwinian standpoint the parallelism between embryos and
fossil remains, so recently pointed out by Professor Agassiz,
is not a mystery, but a necessity ; and the science of Embry-
ology becomes linked on the one hand to Taxonomy, and on
the other to Palaeontology.
In a similar manner the geographical distribution of
animals and plants, the relations of the faunas and floras of
islands to those of the mainland, and the relation of existing
to extinct faunas and floras of the same area, all become
explicable by the aid of the descent theory, and are meaning-
less on any other hypothesis. But time presses, and I will
only pause here to point out the somewhat interesting cir-
cumstance that the facts of this nature, which first led Darwin
to speculate on the origin of species, have been left for their
complete co-ordination and generalisation to the contemporary
founder of modern evolution, Mr. A. E. Wallace, whose works
on the 'Geographical Distribution of Animals' and 'Island
Life' may be regarded as the completion of the twelfth and
thirteenth chapters of the 'Origin of Species.'
Of other classes of facts explained by the principle of evolu-
tion I need only mention the existence of rudimentary organs,
which are, on this view, the surviving remnants of structures
that were useful to the organism at some former period of its
existence. The next classes of cases to which I will allude
are those of "persistent types," and of retrograde develop-
ment or "degeneration." It must be remembered that
Darwin's theory does not postulate, as is so frequently
assumed, the continual advancement of every living creature.
Transformation only takes place when stimulated by the
action of the environment. Under certain conditions, where
by isolation or by constancy in the external conditions of life