84 
The Presidential Address. 
ontogeny, as Haeckel expresses it, is the racial development 
or pliylogeny in mice. 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, hut 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. R. 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 
