An image of a postcard mailed in July 1915 from George Brock Chisholm was included on the "Step Short" website which looks at the first World War experience in Folkestone, England.
The postcard was mailed by Chisholm to his grandmother, Mrs. G. B. Chisholm, Oakville, Ontario.
The original postcard may be viewed here:
Chisholm had a distinguished career. He graduated in Medicine from the University of Toronto by 1924. He served as Director-General of the World Health Organization from 1948-53. His story is detailed on Wikipedia - here:
According to this source, he was initially in the 15th Battalion CEF, but later became an Officer, and received the Military Cross and Bar. During World War Two he served as a psychiatrist, and became Director General of Medical Services in the Canadian Army.
He married Grace McLean Ryrie in 1924, and had two children. He died in 1971 in Victoria, British Columbia