Having completed Climate and Time, Dr. Croll desired to abandon the study of climatology and physics in order to engage in the investigation of the philosophy of evolution preparatory to a work on that subject which he contemplated. But his book had become a topic of general discussion in which he was obliged to participate, to the extent at least of seeing that his views were correctly quoted and understood; and consequently he still published articles and wrote much in correspondence on subjects treated in it. During the summer of 1880 he suffered a strain in the region of the heart which almost completely disabled him. Making an external application of aconite, under his physician's advice, to the seat of the pain, he lost the power of speech temporarily, and contracted an impediment in utterance which did not disappear for several years. He finally thought it his duty to resign his position on the Geological Survey, and accordingly retired from the Government service in the spring of 1881. Application was made by his friends for a liberal pension, corresponding with the merits of his work, under the superannuation act; but although the Government dealt at the time with considerable liberality with other persons whose deserts were no greater than his, an allowance of £100 from the Queen's bounty and a superannuation allowance of £75 16s. 8d a year were all that were granted him; and although repeated applications were made on various aspects of the case, and backed by what would appear to be among the most influential names in the kingdom, no more could be got.
Among the first fruits of Dr. Croll's new studies was the article on Evolution by Force Impossible; a New Argument against Materialism, which was published in the British Quarterly Review for January, 1883. It was a discussion of the question of "What determines molecular motion and force?" and an attempt to prove that force alone, motion alone, or any sort of act alone, that is, undetermined to any particular direction, is unable to account for evolution. If his conclusion were correct, it would follow that Mr. Spencer's theory of evolution by force was absolutely impossible. A copy of this article was sent, with a friendly letter, to Mr. Herbert Spencer, who answered, in substance, that he had not undertaken to deal with the ultimate cause, which he had alleged to be unknowable.
All the papers Mr. Croll had written on climatic subjects since the publication of Climate and Time in 1875 were collected, revised down to date, and republished at the end of 1885 under the title of Discussions on Climate and Cosmology. "This completed his work on geology and physics, and he quietly but deliberately closed his reading and writing on those subjects, which had engrossed his attention for a period of twenty-five years, in which he
