It is quite true that it is, in general, very unsafe to infer the absence of any animal forms during a certain geological period, because no remains of them {106} have as yet been found in the strata then deposited: but in the case of the Cetacea it is safe to do so; for, as Sir Charles Lyell remarks,[100] they are animals, the remains of which are singularly likely to have been preserved had they existed, in the same way that the remains were preserved of the Ichthyosauri and Plesiosauri, which appear to have represented the Cetacea during the secondary geological period.
[Illustration: SKELETON OF A PLESIOSAURUS.] As another example, let us take the origin of wings, such as exist in birds.

Here we find an arm, the bones of the hand of which are atrophied and reduced in number, as compared with those of most other Vertebrates.
Now, if the wing arose from a terrestrial or subaerial organ, this abortion of the bones could hardly have been serviceable--hardly have preserved individuals in the struggle for life.

If it arose from an aquatic organ, like the wing of the penguin, we have then a singular divergence from the ordinary vertebrate fin-limb.

In the ichthyosaurus, in the plesiosaurus, in the whales, in the porpoises, in the seals, and in others, we have shortening of the bones, but no reduction in the number either of the fingers or of their joints, which are, on the contrary, multiplied in Cetacea and the ichthyosaurus.

And even in the turtles we have eight carpal bones and five digits, while no finger has less than two phalanges.