While Gegenbaur was at one with all "pure" morphologists, whether evolutionary or pre-evolutionary, in minimising as far as possible the importance of function in the study of form, he was too cautious and sober a thinker not to recognise the immense part which function really plays. Thus he classified organs, according to their function, into those that established relations with the external world and those that had to do with nutrition and reproduction, very much as Bichat had done before him. Like Darwin, Haeckel and most evolutionists, he interpreted the homological resemblances of animals as being due to heredity, their differences as due to adaptation,[383] but he did not adopt Haeckel's crude and shallow definition of these terms. For Gegenbaur heredity was a convenient expression for the fact of transmission, and was not explained offhand as the mere mechanical result of a certain material structure handed down from germ to germ. Adaptation he defined in a way which took the fullest account of function, and was as far as possible removed from Haeckel's definition of it as the direct mechanical effect of the environment upon the organism. "The organism is altered," writes Gegenbaur, "according to the conditions which influence it. The consequent _Adaptations_ are to be regarded as gradual, but steadily progressive, changes in the organisation, which are striven after during the individual life of the organism, preserved by transmission in a series of generations, and further developed by means of natural selection. What has been gained by the ancestor becomes the heritage of the descendant. Adaptation and Transmission are thus alternately effective, the former representing the modifying, the latter the conservative principle.... Adaptation is commenced by a change in the function of organs, so that the _physiological relations_ of organs play the most important part in it. Since adaptation is merely the material expression of this change of function, the modification of the function as much as its expression is to be regarded as a gradual process. In Adaptation, the closest connection between the function and the structure of an organ is thus indicated. Physiological functions govern, in a certain sense, structure; and so far what is morphological is subordinated to what is physiological" (_Elements_, pp. 8-9). Gegenbaur recognised also that morphological differentiation depended largely on the physiological division of labour (_Grundzuege_, p. 49).