Mr Balfour was a younger son of the late Mr J. M. Balfour of Whittinghame, near Prestonkirk, and of the late Lady Blanche Balfour, a sister of Lord Salisbury. He entered at Trinity College, Cambridge, from Harrow, in October 1870. He brought from school the reputation of being a clever boy, whom the masters liked and respected, but of not sufficient ability to distinguish himself remarkably at Cambridge. Those who expressed this opinion overlooked the fact that he had already evinced a decided bent for Natural Science, and had published a brief memoir on the geology of his native county, Haddingtonshire. In his very first term he was fortunately induced to attend the biological lectures of the Trinity Prælector in Physiology, Mr Michael Foster; he made rapid progress, and at Easter 1871 he obtained the Natural Science Scholarship at Trinity College. He at once commenced original research in the direction in which he was afterwards to be so distinguished; and after two years’ work published a paper on _The Development of the Chick_ in the _Microscopical Journal_ for July, 1873. Indeed, we believe that the time spent on this and kindred investigations diminished somewhat the brilliancy of his degree, for he was placed second instead of first, as had been expected, in the Natural Sciences Tripos of 1873. In November of that year he was nominated by the Board of Natural Science Studies to work at the Zoological Station at Naples, then lately established by Dr Anton Dohrn. His object in going there was to continue his investigations on Development, and before starting he had determined to study the Elasmobranch Fishes (Sharks and Rays), as it seemed likely, from their pristine characters, that their development would throw great light on the early history of vertebrate animals. The result showed how wisely he had made his selection. He made discoveries of the highest value in reference to the development of certain organs, and the origin of the nerves from the spinal cord—points which had baffled the most acute previous observers. These were not merely valuable for the history of the special group from which they were derived, but threw a flood of light upon the connexion between vertebrates and invertebrates, and their derivation from a common ancestry; views which he expanded afterwards in his work on Embryology. The results of his Neapolitan researches were embodied in the dissertation upon which he rested his candidature for a Fellowship at Trinity College; and were afterwards printed in the _Philosophical Transactions_ for 1875. Fortunately for him, a Natural Science Fellowship was vacant in 1874, to which he was elected, in consequence of the value of this dissertation. It is what is called an open secret that its great merits were at once recognized by Professor Huxley, to whom it had been referred.