The peculiarity of six fingers on the hand and six toes on the feet... is then sometimes seen to descend through several generations. It was Mr. Lawrence's opinion that, a pair, in which both parties were so distinguished, might be the progenitors of a new variety of the race who would be thus marked in all future time. It is not easy to surmise the causes which operate in producing such varieties. Perhaps they are simply types in nature, possible to be realized under certain appropriate conditions, but which conditions are such as altogether to elude notice.

Note: Chambers and his brother William were both born with this condition. Robert was made lame by the operation to remove the sixth digits from his feet. - Vestiges of the Natural History of Creation (1844)