James Cossar Ewart was born in Penicuik, Midlothian, on 26 November 1851. He was educated in Penicuik and entered the University of Edinburgh as a medical student in 1870, graduating as a Bachelor of Medicine and Master of Surgery in 1874.
Ewart found employment in London as Curator of the Zoological Museum at University College, and also published a number of papers on the structure of the retina and lens, the sexual organs of the lamprey, and the placentation of the Shanghai River deer. His research on Bacillus anthracis was presented as a thesis for the degree of Doctor of Medicine at Edinburgh.
After a brief return to Edinburgh as Lecturer in Anatomy in the Extra-Mural School, Ewart was appointed to the Chair of Natural History in the University of Aberdeen at the end of 1878. It was there that Ewart became interested in marine biological investigation and established an experimental station on the coast in the area, the first of its kind in Britain. In 1882 Ewart secured the post of Regius Professor of Natural History at the University of Edinburgh, a position he was to hold for 45 years.
In around 1894, Ewart began his investigations into experimental breeding, the work for which he was to become best known. He was especially concerned with disproving the long-held theory of Telegony, which held that a sire may 'infect' the dam he serves so as to influence the characteristics of future offsprings from different sires. To do this, Ewart repeated a classic experiment which supposedly proved this theory, the case involving the Arab mare belonging to Lord Morton which produced a striped foal after mating with a quagga, even when subsequently served by an Arab stallion. Ewart's experiment used a Burchell's zebra (the quagga having become extinct), but found that the 'subsequent foals' showed no signs of having been affected by a previous zebra sire. The results of his work were published in The Penycuik Experiments (1899). Ewart's preoccupation with the evolution of horses, particularly the theories that early horses were striped and that the modern domestic horse had a multiple origin, was to occupy him for large part of his career.
Following his major publications on horses, Ewart turned his attention to experiments on sheep, being largely occupied with cross-breeding for fleece improvements, travelling as far as Australia and New Zealand to advise sheep breeders and related institutions there. Ewart's later work focused on the origin and history of feathers in birds and their relation to scales in reptiles. The rearing of penguins at the then fairly new Edinburgh Zoo provided him with the relevant material.
Ewart remained adamant that animal breeding should be taken seriously by universites and funding bodies, both for its academic importance in terms of the emergent science of genetics as well as its practical and financial use to agriculturalists around the world. It was certainly at least partly due to Ewart's knowledge, reputation and advocacy that a University Lectureship in Genetics, the first post of its kind in the UK, was instituted in 1911 at the University of Edinburgh.
James Cossar Ewart died at his home in Penicuik on New Year's Eve 1933 after a short illness. He was elected a Fellow of the Royal Society in June 1893, having jointly delivered the Croonian Lecture in 1881, jointly with George John Romanes. F.H.A Marshall described him as not only 'a distinguished man of science but also as a delightful companion, a kindly and courteous host, and a loyal and loveable friend.'