Darwin notes the situational irony that many islands do not have any frog population, despite the fact that islands typically have a perfect climate to support frogs. He goes on to explain how this situational irony is a paradox for which his theory makes room:
But as these animals and their spawn are immediately killed (with the exception, as far as known, of one Indian species) by sea-water, there would be great difficulty in their transportal across the sea, and therefore we can see why they do not exist on strictly oceanic islands. But why, on the theory of creation, they should not have been created there, it would be very difficult to explain.
Darwin explains that while it seems strange at first that frogs are so rarely found on oceanic islands, it in fact makes sense once one considers the dispersal of frog species. Darwin has already demonstrated the unlikelihood that every species develops independently, so any frogs that do inhabit islands are likely related to one another. This means that they would not have developed in place, but would rather have reached the islands through wide distribution. As a general rule, while frogs thrive in the conditions furnished by oceanic islands, they and their spawn die if they come into contact with salt water. This means that they would only have been able to reach any given island when that island was part of a larger land mass; otherwise, they would have had to cross the ocean, which they cannot do. It thus fits Darwin's theory for frogs to be rarely found on oceanic islands.
On the other hand, Darwin points out, if a supreme being has created each species individually, there is no reason whatsoever for frogs to be absent from oceanic islands. As he does elsewhere, Darwin is careful not to insult the idea of God. Rather, he casts aspersions on people who believe God would have done something so nonsensical. Darwin's theory makes room for God to exist and furthermore to make more logical decisions than the theory of creation suggests.
