In a rough-and-ready fashion, Huxley’s active life may be broken into a set of decennial periods, each with tolerably distinctive characters.  The first period, roughly from 1850 to 1860, was almost purely scientific.  It was occupied by his voyage, by his transition to science as a career, his researches into the invertebrate forms of life, the beginning of his palaeontological investigations, and a comparatively small amount of lecturing and literary work.  The second decennium still found him employed chiefly in research, vertebrate and extinct forms absorbing most of his attention.  He was occupied actively with teaching, but the dominant feature of the decennium was his assumption of the Darwinian doctrines.  In connection with these latter, his literary and lecturing work increased greatly, and the side issues of what was, in itself, purely a scientific controversy began to lead him into metaphysical and religious studies.  The third period, from 1870 to 1880, was considerably different in character.  He had become the most prominent man in biological science in England, at a time when biological science was attracting a quite unusual amount of scientific and public attention.  Public honours and public duties, some of them scientific, others general, began to crowd upon him, and the time at his disposal for the quiet labours of investigation became rapidly more limited within this period.  He was secretary of the Royal Society, a member of the London School Board, president of the British Association, Lord Rector at several universities, member of many royal commissions, government inspector of fisheries, president of the Geological Society.  In this multitude of duties it was natural that the bulk of strictly scientific output was limited, but, on the other hand, his literary output was much larger.  Between 1880 and 1890 he had reached the full maturity of a splendid reputation, and honours and duties pressed thick upon him.  For part of the time he was president of the Royal Society, the most distinguished position to which a scientific man in England can attain, and he was held by the general public at least in as high esteem as by his scientific contemporaries.  A small amount of original scientific work still appeared from his pen, but he was occupied chiefly with more general contributions to thought.

[Illustration:  CARICATURE OF HUXLEY DRAWN BY HIMSELF Reproduced by permission from Natural Science, vol. vii., No. 46]

Throughout his life, Huxley had never been robust.  From his youth upwards he had been troubled by dyspepsia with its usual accompaniment of occasional fits of severe mental and physical depression.  In 1872 he was compelled to take a long holiday in Egypt, and, although he returned to resume full labour, it is doubtful if from that time onwards he recovered even the strength normal to him.  In 1885, his ill-health became grave; in the following years he had two attacks of pleurisy, and symptoms of cardiac mischief became