Physiologists believe that the brain must be bathed by warm blood to be highly active, and this requires aerial respiration; so that warm-blooded mammals when inhabiting the water lie under a disadvantage in having to come continually to the surface to breathe.

With fishes, members of the shark family would not tend to supplant the lancelet; for the lancelet, as I hear from Fritz Muller, has as sole companion and competitor on the barren sandy shore of South Brazil, an anomalous annelid.

The three lowest orders of mammals, namely, marsupials, edentata, and rodents, co-exist in South America in the same region with numerous monkeys, and probably interfere little with each other.