Ishmael entertains two questions in this chapter. First, he wonders if whales have increased or decreased in size since antiquity. To this, he has no easy answer, since on the one hand, he notes that many animals, like humans, have grown larger as the years have progressed. But he also notes ancient Roman accounts of whales that would have them being roughly 300 feet long, compared to about 100 feet for the largest contemporary sperm whales. Second, Ishmael considers whether whaling practices could cause the species to go extinct. Although he says it is reasonable to worry about just such a matter, he believes that the rate of whale-hunting is not high enough to jeopardize seriously the number of whales in the world—and that the sperm whale will “outlive” many of the rest of the species currently alive on the planet (including, perhaps, humans).
Ishmael enters this chapter with a legitimate scientific question, and is unable to answer it one way or another. Some accounts show that whales have gotten larger—others, that whales have decreased in size. Even evidence can't be trusted. Ishmael's consideration of whether whaling could lead to extinction of whales is interesting both in that he would contemplate such a possibility and because his answer turned out to be so very wrong.