Didn’t happen. See the census reports from 1860 and 1870.
Sorry I can’t give you more exact info than that. The author was casting a critical eye on precisely those census figures -— trying to get better estimates from other documentary/testimentary sources. Bah. I read so much IO can’t remember where I read it. But I think it was with in the last month. Possibly — possibly - a book review in the Guardian (UK).