i6 
SCIENCE. 
intellectual as in the physical world. A theory is a species 
of thinking, and its right to exist is coextensive with its 
power of resisting extinction by its rivals. 
From this point of view it appears to me that it would be 
but a poor way of celebrating the Coming of Age of the Ori- 
gin of Species were I merely to dwell upon the facts, un- 
doubted and remarkable as they are, of its far-reaching in- 
fluence and of the great following of ardent disciples who 
are occupied in spreading and developing its doctrines. 
Mere insanities and inanities have before now swollen to 
portentous size in the course of twenty years. Let us rather 
ask this prodigious change in opinion to justify itself; let 
us inquire whether anything has happened since 1859 which 
will explain, on rational grounds, why so many are worship- 
ping that which they burned, and burning that which they 
worshipped. It is only in this way that we shall acquirethe 
means of judging whether the movement we have witnessed 
is a mere eddy of fashion, or truly one with the irreversible 
current of intellectual progress, and, like it, safe from retro- 
gressive reaction. 
Every belief is the product of two factors : the first is the 
state of the mind to which the evidence in favor of that be- 
lief is presented ; and the second is the logical cogency of 
the evidence itself. In both these respects the history of 
biological science during the last twenty years appears to 
me to afford an ample explanation of the change which has 
taken place; and a brief consideration of the salient events 
of that history will enable us to understand why, if the 
“Origin of Species” appeared now, it would meet with a 
very different reception from that which greeted it in 1859. 
One-and-twenty years ago, in spite of the work commenced 
by Hutton, and continued with rare skill and patience by 
Lyell, the dominant view of the past history of the earth was 
catastrophic. Great and sudden physical revolutions, whole- 
sale creations and extinctions of living beings, were the or- 
dinary machinery of the geological epoch brought in fashion 
by the misapplied genius of Cuvier. It was gravely main- 
tained and taught that the end of every geological epoch 
was signalized by a cataclysm, by which every living being 
on the globe was swept away, to be replaced by a brand-new 
creation when the world returned to quiescence. A scheme 
of nature which appeared to be modelled on the likeness of 
a succession of rubbers of whist, at the end of each of which 
the players upset the table and called for a new pack, did 
not seem to shock anybody. 
I may be wrong, but I doubt if at the present time there is 
a single responsible representative of these opinions left. 
The progress of scientific geology has elevated the funda- 
mental principle of uniformitarianism, that the explanation 
of the past is to be sought in the study of the present, into 
the position of an axiom ; and the wild speculations of the 
catastrophists, to which we all listened with respect a quarter 
of a century ago, would hardly find a single patient hearer 
at the present day. No physical geologist now dreams of 
seeking outside the ranges of known natural causes for the 
explanation of anything that happened millions of years ago, 
any more than he would be guilty of the like absurdity in 
regard to current events. 
The effect of this change of opinion upon biological specu- 
lation is obvious. For, if there have been no periodical 
general physical catastrophes, what brought about the as- 
sumed general extinctions and re-creations of life which are 
the corresponding biological catastrophes ? And if no such 
interruptions of the ordinary course of nature have taken 
place in the organic, any more than in the inorganic world, 
what alternative is there to the admission of Evolution? 
The doctrine of Evolution in Biology is the necessary re- 
sult of the logical application of the principles of uniformi- 
tarianism to the phenomena of life. Darwin is the natural 
successor of Hutton and Lyell, and the “ Origin of Species” 
the natural sequence of the “ Principles of Geolcgy.” 
The fundamental doctrine of the “ Origin of Species,” as 
of all forms of the theory of Evolution applied to biology, is 
“ that the innumerable species, genera, and families of or- 
ganic beings with which the world is peopled have all de- 
scended, each within its own class or group, from common 
parents, and have all been modified in the course of de- 
scent.” 1 
1 “ Origin of Species,” ed. 1, p. 457. 
And, in view of the facts of geology, it follows that all 
living animals and plants “ are the lineal descendants of 
those which lived long before the Silurian epoch.” 1 
It is an obvious consequence of this theory of Descent 
with Modification, as it is sometimes called, that all plants 
and animals, however different they may now be, must, at 
one time or other, have been connected by direct or indirect 
intermediate gradations, and that the appearance of isola- 
tion presented by various groups of organic beings must be 
unreal. 
No part of Mr. Darwin’s work ran more directly counter 
to the prepossessions of naturalists twenty years ago than 
this. And such prepossessions were very excusable, for there 
was undoubtedly a great deal to be said, at that time, in 
favor of the fixity of species and of the existence of great 
breaks, which there was no obvious or probable means of 
filling up, between various groups of organic beings. 
For various reasons, scientific and unscientific, much 
had been made of the hiatus between man and the rest of 
the higher mammalia, and it is no wonder that issue was 
first joined on this part of the controversy. I have no 
wish to revive past and happily forgotten controversies, 
but I must state the simple fact that the distinctions in 
cerebral and other characters, which were so hotly affirmed 
to separate man from all other animals in i860, have all 
been demonstrated to be non-existent, and that the contrary 
doctrine is now universally accepted and taught. 
But there were other cases in which the wide structual 
gaps asserted to exist between one group of animals and 
another were by no means fictitious; and, when such structual 
breaks were real, Mr. Darwin could account for them only 
by supposing that the intermediate forms which once ex- 
isted had become extinct. In a remarkable passage he 
says : — 
“ We may thus account even for the distinctness of whole 
classes from each other — for instance of birds from all 
other vertebrate animals — by the belief that many animal 
forms of life have been utterly lost, through which the 
early progenitors of birds were formerly connected with 
the early progenitors of the other vetebrate classes.” 2 
Adverse criticism made merry over such suggestions as 
these. Of course it was easy to get out of the difficulty by 
supposing extinction ; but where was the slightest evidence 
that such intermediate forms between birds and reptiles as 
the hypothesis required ever existed? And then probably 
followed a tirade upon this terrible forsaking of the paths 
of “ Baconian induction.” 
But the progress of knowledge has justified Mr. Darwin 
to an extent which could hardly have been anticipated. In 
1862, the specimen of Archaeopteryx , which until the last 
two or three years has remained unique, was discovered ; 
and it is an animal which, in its feathers and the greater 
part of its organization, is a veritable bird, while, in other 
parts, it is as distinctly reptilian. 
In 1868, I had the honour of bringing under your notice, 
in this theatre, the results of investigations made, up to 
that time, into the anatomical characters of certain ancient 
reptiles, which showed the nature of the modifications in 
virtue of which the type of the quadrupedal reptile passed 
into that of the bipedal bird ; and abundant confirmatory 
evidence of the justice of the conclusions which I then 
laid before you has since come to light. 
In 1875, the discovery of the toothed birds of the cre- 
taceous formation in North America, by Prof. Marsh, com- 
pleted the series of transitional forms between birds and 
reptiles, and removed Mr. Darwin’s proposition that 
“ many animal forms of life have been utterly lost, through 
which the early progenitors of birds were formerly connect- 
ed with the early progenitors of the other vertebrate 
classes,” from the region of hypothesis to that of demon- 
strable fact. 
In 1859, there appeared to be a very sharp and clear 
hiatus between vertebrated and invertebrated animals, not 
only in their structure, but, what was more important, in 
their development. I do not think that we even yet know 
the precise links of connection between the two ; but the 
investigations of Kowalewsky and others upon the develop- 
1 “ Origin of Species,” ed. I, p. 458. 
2 “ Origin of Species,” ed. 1, p. 431. 
