Updated: Jan 31
In England, lace makers used the toy version of the English bull dog as lap warmers while they worked. When the lace industry moved to France, they took their dogs with them. There, the English bulldogs probably bred with terriers to create bouledogues français, or French bulldogs.
Italicized extract from 11/18/18 article on French bulldogs by Rebecca OConnell in MentalFloss.com
Pic of French bulldog from NYPost.com