is clear, that though the earlier life-periods of the earth’s - 
history may be for ever hidden from us, the period of which we 
have actual record is sufficiently long to make it certain that 
we must have in that period many common progenitors of exist- 
ing species, or of species which came into existence in the later 
epochs of Geology. We should, therefore, expect to meet, as 
palaeontologists, with numerous directly intermediate types; 
and the very general absence of such appears to me to be, to 
begin with, a very serious obstacle in the way of the Darwinian 
hypothesis. 
2. The Lapse of Geological Time. — The argument under 
this head I may pass over without discussion. As a field- 
geologist, I am fully prepared to admit the vastness of geolo- 
gical time; hut I do not see that we have at present any 
sufficiently definite data by which we can estimate whether this 
time has, or has not, been sufficient to allow of the production 
of all living and extinct species of animals and plants by the 
action of natural selection. Geological time, as asserted by 
Mr. Darwin, is no doubt commonly underestimated ; but we 
cannot at present even approximately determine how long a 
period has elapsed since the first introduction of living beings 
upon the globe, and we have not the smallest means of calcu- 
lating how long a period would be required for the origin of 
species on Darwinian principles. It seems futile, therefore, to 
attempt to draw any conclusion from the comparison of two 
unknown quantities. Sir William Thomson’s conclusions, if 
proved, would undoubtedly seriously affect the position of the 
Darwinian theory, but it cannot be said that they are certain, 
and it seems better at present to regard our knowledge as in- 
sufficient for the formation of any definite opinion on this subject. 
3. The Poorness of our Palaeontological Collections. — The 
next section of Mr. Darwin’s argument deals with the poverty 
of our best palaeontological collections, which he evidently 
regards as so great as to render all negative evidence, founded 
on the absence of certain forms of life, as of no value when 
opposed to his theoretical views. Unquestionably if we were 
to take our entire palaeontological collections and compare 
them with the vast number of animals and plants, which we 
may infer from various considerations to have existed in past 
time, but of which we have now no traces, Mr. Darwin is 
they are necessarily regarded as distinct genera. Hence, they do not lead 
us any nearer to the graduated series of transitional forms, which will 
have to he found before we can positively assert that Equus is a lineal 
descendant of Hipparion, and the latter of Anchitherium. 
