Science Marches On: The exotic wildlife is thought to have come to the island by way of now-sunken land bridges. At the time of the movie's release, continental drift was considered fringe science.
Values Dissonance: Humans showing up in a pristine natural habitat and promptly capturing a (baby!) elephant, a monkey, an ostrich and a bunch of other animals just does not have the same ring to it as it did when this movie was made, with nearly everything going extinct and huge problems with animal trafficking.
Especially depressing with the elephant. When young Francis meets a baby elephant for the first time, he isn't even stunned, as any normal child would be. The first idea that comes to his mind is to figure out a way to tie it up and use it to carry stuff, to make it profitable.