78 SECTIONAL ADDRESSES. 
geographical varieties closely and extensively, will smile at the conception 
of the origin of species per saltum.’ 
The age of the earth, as estimated by Lord Kelvin and Prof. Tait, was 
one of Darwin’s ‘ sorest troubles.’ ‘I should rely much on pre-Silurian 
times,’ he wrote in 1871, ‘ but then comes Sir W. Thomson, like an odious 
spectre.’ Lord Salisbury’s treatment of this subject in his address at 
Oxford in 1894 will be remembered by many. Entirely accepting the 
fact that Darwin had ‘ disposed of the doctrine of the immutability of 
species,’ he ridiculed the demands which evolution by Natural Selection 
makes upon the bank of time. ‘Of course if the mathematicians are 
right, the biologists cannot have what they demand. If, for the purposes 
of their theory, organic life must have existed on the globe more than a 
hundred million years ago, it must, under the temperature then prevailing, 
have existed in a state of vapour. The jelly-fish would have been dissi- 
pated in steam long before he had had a chance of displaying the 
advantageous variation which was to make him the ancestor of the 
human race.’ I venture to refer to this difficulty, although a difficulty 
no longer, because it provides a good illustration of the help which so 
often comes to us at these meetings, and also recalls a vigorous personality, 
our kindly Treasurer for many years, Prof. John Perry. Walking together 
on the Sunday of the Leeds Meeting in 1890 he explained to me the 
evidence on which Thomson and Tait had relied, and said that he believed 
the argument founded on the cooling of the earth to be sound. When, 
however, he heard Lord Salisbury’s address four years later, and decided 
to re-examine the evidence, he soon discovered that an important considera- 
tion had been overlooked. With his kind help I chose this subject, together 
with the biological evidence for the age of the habitable globe, for my 
address at Liverpool in 1896. In the following year as we were travelling 
across Canada after the Toronto Meeting and the chance of collecting 
insects for a few minutes at each station could not be resisted, Lord Kelvin 
said to his wife: ‘ My dear, I think we must forgive Poulton for thinking 
that the earth is so very old when he works so hard in one day out of all 
the endless millions of years in which he believes!’ A quarter of a century 
later ‘the Age of the Earth’ was the subject of a joint discussion at 
Edinburgh, when the Thomson-Tait limitation of time was abandoned in 
consequence of researches on radioactivity. 
We now come to biological criticisms of evolution by Natural Selection, 
especially those urged by my friend Sir John Farmer in his presidential 
address to the Botanical Section at Leicester in 1907,11 and concisely 
restated in 1927.12, In the latter publication the theory of evolution as it 
was held forty years ago, and, I may add, very nearly asit is held to-day, 
was described as ‘the notion that the basis of evolutionary change in 
living forms lay in the gradual summation of almost imperceptibly small 
variations, and that, in fact, specific change was attributable to selection 
41 An answer to the criticisms in this address appeared in the Introduction to 
Essays on Evolution, Oxford, 1908, p. xliv. 
2 Prac. Roy. Soc., B., vol. 101, 1927, pp. i, ii. 
