TRANSACTIONS OF SECTION D. 3 
of this Association when it last met in this city that biologists have no right to 
take part in this discussion. In his Anniversary Address to the Geological So- 
ciety in 1869 Huxley said: ‘ Biology takes her time from geology. ... If the 
geological clock is wrong, all the naturalist will have to do is to modify his 
notions of the rapidity of change accordingly.’ This contention is obviously true 
as regards the time which has elapsed since the earliest fossiliferous rocks were laid 
down. For the duration of the three great periods we must look to the geologist ; 
but the question as to whether the whole of organic evolution is comprised within 
these limits, or, if not, what proportion of it is so contained, is a question for the 
naturalist. The naturalist alone can tell the geologist whether his estimate is sufli- 
cient, or whether it must be multiplied by a small or by some unknown but cer- 
tainly high figure, in order to account for the evolution of the earliest forms of 
life known in the rocks, This, I submit, is a most important contribution to the 
discussion. 
Before proceeding further it is right to point out that obviously these argu- 
ments will have no weight with those who do not believe that evolution is a 
reality. But although the causes of evolution are greatly debated, it may be 
assumed that there is no perceptible difference of opinion as to evolution itself, and 
this common ground will bear the weight of all the zoological arguments we shall 
consider to-day. 
It will be of interest to consider first how the matter presented itself to 
naturalists before the beginning of this controversy on the age of the habitable 
earth. I will content myself with quotations from three great writers on biological 
problems—men of extremely different types of mind, who yet agreed in their 
conclusions on this subject. 
In the original edition of the ‘ Origin of Species’ (1859), Darwin, arguing from 
the presence of trilobites, Nautilus, Lingula, &c., in the earliest fossiliferous rocks, 
comes to the following conclusion (pages 306, 307) : ‘Consequently, if my theory 
be true, it is indisputable that before the lowest Silurian stratum was deposited 
long periods elapsed, as long as, or probably far longer than, the whole interval 
from the Silurian age to the present day; and that during these vast yet quite 
unknown periods of time the world swarmed with living creatures.’ 
The depth of his conviction in the validity of this conclusion is seen in the fact 
that the passage remains substantially the same in later editions, in which, how- 
ever, Cambrian is substituted for Silurian, while the words ‘ yet quite unknown’ 
are omitted, as a concession, no doubt, to Lord Kelvin’s calculations, which he 
then proceeds to discuss, admitting as possible a more rapid change in organic life, 
induced by more violent physical changes.’ 
We know, however, that such concessions troubled him much, and that he 
was really giving up what his judgment still approved. Thus he wrote to 
Wallace on April 14, 1869: ‘Thomson’s views of the recent age of the world have 
been for some time one of my sorest troubles... .’ And again, on July 12, 
1871, alluding to Mivart’s criticisms. he says: ‘I can say nothing more about 
missing links than what I have said. JI should rely much on pre-Silurian times ; 
but then comes Sir W. Thomson, like an odious spectre.’ 
Huxley’s demands for time in order to account for pre-Cambrian evolution, as 
he conceived it, were far more extensive. Although in 1869 he bade the 
naturalist stand aside and take no part in the controversy, he had nevertheless 
spoken as a naturalist in 1862, when, at the close of another Anniversary Address 
to the same Society, he argued from the prevalence of persistent types ‘that any 
admissible hypothesis of progressive modification must be compatible with per- 
sistence without progression through indefinite periods;’ and then maintained 
that ‘should such an hypothesis eventually be proved to be true . . . the conclusion 
will inevitably present itself that the Palseozoic, Mesozoic, and Cainozoic faune 
sophy, vol. i. part 2, second edition; and ‘On the Secular Cooling of the Earth,’ 
Royal Society of Edinburgh, 1862. 6th ed. 1872, p. 286. 
D2 
