Amphibians are still linked to the water by their need for a fluid medium where the egg and sperm can unite, i.e. external fertilization. That is why toads, tree frogs etc return to ponds to breed each spring. The penis is the liberating development that frees creatures from the need to immerse themselves to reproduce and permits the utilization of habitats far from bodies of water large enough to immerse the reproducing animal. Or at least I like to see it that way, penises get little good pub these days.

Layton Runkle