The family is descended from Rowland Curtois, who was Mayor of Lincoln in 1670. He had one son, John, who in turn had two sons Rowland (the elder) and John. From Rowland is descended the elder branch of the family, who are still around, and from John were descended the younger branch, who have now died out.
The first Rowland's origins are not certain, though are probably known and are shown in a separate tree. There are also many other Curtois and Curtis's around Lincolnshire in this period - some of them may be related, and when I get the chance I'll put them on here too.