With elephants recognized as the slowest breeders of all known animals, Darwin made a laborious calculation, similar to the bacterial calculation above but more detailed, assuming that elephants started breeding at age 30 and continued until age 90, producing 6 young in that time.
Of course he had no computers, nor calculators, and apparently kept track of 90 or more age classes and made his calculations on paper. He calculated by hand on paper and alas those notes have never been found. But he said it cost him “some pain” to reach the conclusion that at the end of the fifth century, fifteen million elephants would be walking the earth, descended from one original pair. From this, he concluded that unlimited growth is impossible.
There is no exception to the rule that every organic being naturally increases at so high a rate that, if not destroyed, the earth would soon be covered by the progeny. — Charles Darwin, 1859
That, he explained in Chapter Three of his Origin of Species. After explaining results of selection by people in the breeding of domestic animals, he introduced the concept of selection by natural causes in the wild, which he called “natural selection.” The simplest model of unlimited population growth was thus useful in the extreme, leading to an inviolable law of biology and the theory of evolution as one of its consequences. Individuals with qualities that allow them to suffer lower mortality or to reproduce slightly faster, and who pass those qualities to their offspring, will be the ones whose qualities predominate.