Either of my last methods, applied to Mr. Darwin’s supposition of twin births alone at intervals of 30 years, would give the same results as the series 1, 1, 2, 4, 7, 13, &c. (continued by the scale 2+0+0-1), whose terms quadrupled give, as Mr. George¹ observes, the elephants alive, on that supposition, in the decade after each bearing year.² In the periods of 20 years, however, between these exceptional decades, their numbers are only the doubles of these terms. The general term of this progression being 1:839, shows the period of doubling to be about 34 years, while, on my truer supposition of single births at intervals of 12 years, the increase is about a sixteenth faster. The mistake that led me at first to a doubling in 23 years may be the same as the original calculator’s. At least it would fully account for his result.

E.L.G.