单选题 .  ①During the Pleistocene epoch, several species of elephants isolated on islands underwent rapid dwarfing. ②This phenomenon was not necessarily confined to the Pleistocene, but may have occurred much earlier in the Southeastern Asian islands, although evidence is fragmentary. ③Several explanations are possible for this dwarfing. ④For example, islands often have not been colonized by large predators or are too small to hold viable predator populations. ⑤Once free from predation pressure, large body size is of little advantage to herbivores. ⑥Additionally, island habitats have limited food resources, a smaller body size and a need for fewer resources would thus be favored. ⑦Interestingly, the island rule is reversed for small mammals such as rodents, for which gigantism is favored under insular conditions.43.  The primary purpose of the passage is to ______