Yet no single transitional form has yet been met with in spite of the multitudinous individuals preserved.

Again, with their modern representatives the Cetacea, one or two aberrant forms alone {133} have been found, but no series of transitional ones indicating minutely the line of descent.

This group, the whales, is a very marked one, and it is curious, on Darwinian principles, that so few instances tending to indicate its mode of origin should have presented themselves.

Here, as in the bats, we might surely expect that some relics of unquestionably incipient stages of its development would have been left.
[Illustration: SKELETON OF A PLESIOSAURUS.] The singular order Chelonia, including the tortoises, turtles, and terrapins (or fresh-water tortoises), is another instance of an extreme form without any, as yet known, transitional stages.

Another group may be finally mentioned, viz.