Such adaptations are the transparency and colourlessness of the tissues, and the modifications of the foot, which still shows in Atlanta the form common in Pectin:branchia (compare fig.
Bionomically, metamorphosis may be defined as the sum of adaptations that have gradually fitted the larva (caterpillar or maggot) for one kind of life, the fly for another.
It was the application of Fritz Miller's law of recapitulation which gave the chief stimulus to embryological investigations between 1865 and 1890; and, though it is now recognized that " recapitulation " is vastly and bewilderingly modified by special adaptations in every case, yet the principle has served, and still serves, as a guide of great value.
This process is known as " direct adaptation "; and there is no doubt that such structural adaptations are acquired by an animal in the course of its life, though such changes are strictly limited in degree and rare rather than frequent and obvious.
It has been argued that the elaborate structural adaptations of the nervous system which are the corporeal correlatives of Theory complicated instincts must have been slowly built up by the transmission to offspring of acquired ex perience, that is to say, of acquired brain structure.