any set of ideas brought into the mind without an immediate, almost unconscious, overhauling of the memory for any other ideas at all congruous.  In a strict scientific exposition Huxley would choose from the multitude of possible comparisons that most simple and most intelligible to his audience; when in a lighter vein, he gave play to a natural humour in his choice.  Instances of his method of exposition by comparison abound in his published addresses.  Let us take one or two.  In the course of an address to a large mixed audience so early in his public career as 1854, in making plain to them the proposition, somewhat novel for those days, that the natural history sciences had an educational value, he explained that the faculties employed in that subject were simply those of the common sense of every-day life.

“The vast results obtained by Science are won by no mystical faculties, by no mental processes other than those which are practised by every one of us, in the humblest and meanest affairs of life.  A detective policeman discovers a burglar from the marks made by his shoe, by a mental process identical with that by which Cuvier restored the extinct animals of Montmartre from fragments of their bones.  Nor does that process of induction and deduction by which a lady, finding a stain of a peculiar kind on her dress, concludes that somebody has upset the inkstand thereon, differ in any way, in kind, from that by which Adams and Leverrier discovered a new planet.”

In one of his addresses to working men on Man’s Place in Nature he shewed that from time to time in the history of the world average persons of the human race have accepted some kind of answer to the insoluble riddles of existence, but that from time to time the race has outgrown the current answers, ceasing to take comfort from them.

“In a well-worn metaphor a parallel is drawn between the life of man and the metamorphosis of a caterpillar into a butterfly; but the comparison may be more just as well as more novel, if for its former term we take the mental progress of the race.  History shews that the human mind, fed by constant accessions of knowledge, periodically grows too large for its theoretical coverings, and bursts them asunder to appear in new habiliments, as the feeding and growing grub, at intervals, casts its too narrow skin and assumes another, itself but temporary.  Truly, the imago state of man seems to be terribly distant, but every moult is a step gained, and of such there have been many.”

As another instance, the following from his address on a “Liberal Education” may be taken.  He had been discussing the intellectual advantage to be derived from classical studies, and had been comparing, to the disadvantage of the latter, the intellectual discipline which might be got from a study of fossils with the discipline claimed by the ordinary experts upon education to be the results of classical training.  He wished to anticipate the obvious objection to his argument:  that the subject-matter of palaeontology had no direct bearing on human interests and emotions, while the classical authors were rich in the finest humanity.