It{107} is difficult, then, to believe that the Avian limb was developed in any other way than by a comparatively sudden modification of a marked and important kind.
[Illustration: SKELETON OF AN ICHTHYOSAURUS.] How, once more, can we conceive the peculiar actions of the tendrils of some climbing plants to have been produced by minute modifications?
These, according to Mr.Darwin,[101] oscillate till they touch an object, and then embrace it.

It is stated by that observer, "that a thread weighing no more than the thirty-second of a grain, if placed on the tendril of the _Passiflora gracilis_, will cause it to bend; and merely to touch the tendril with a twig causes it to bend; but if the twig is at once removed, the tendril soon straightens itself.

But the contact of other tendrils of the plant, or of the falling of drops of rain, do not produce these effects."[102] But some of the zoological and anatomical discoveries of late years tend rather to diminish than to augment the evidence in favour of minute and gradual modification.

Thus all naturalists now admit that certain animals, which were at one time supposed to be connecting links between groups, belong altogether to one group, and not at all to the other.

For example, the aye-aye[103] (_Chiromys Madagascariensis_).