Abstract
Neo-Darwinian theory predicts 'finely graduated organic change' (Darwin's own words) between groups of organisms as they descend with modification. If amphibians evolved from fish, then it would be reasonable to assume that, at the ultrastructural level, the skin of a frog would be similar to that of a fish. Skin from Xenopus laevis (frog) and Carassius auratus (goldfish) was harvested and processed for transmission electron microscopy (TEM). These two types of skin are dramatically different in many ways. Frogs use sophisticated structures such as mitochondria and well-designed active ion transport systems in their skin that fish simply do not have. Additionally, fish skin is completely anchored to bone in an elegant manner, using sheets of helically arranged collagen fibres to provide for rapid swimming. Frogs respire mostly through their skin, but they have lungs as well, whereas most fish breathe only through gills.