28 GENERAL PHYSIOLOGY 



and in which the various parts, tissues, and organs consist of 

 specific forms of cells. Although this knowledge carried with it 

 the fact that the cell is the element of the living organism and 

 the place where the life-processes occur, nevertheless, the cell, 

 except in botany and embryology, has not yet been made a subject 

 of special physiological study. We shall see presently that 

 precisely in this direction is to be expected an essential advance 

 in the physiology of the future. 



The third discovery, which thus far has not been fruitful in 

 physiology, is that of descent in the organic world. The theory of 

 descent, sketched in its outlines by Lamarck, and firmly founded 

 by Darwin upon the principle of selection, has produced a great 

 revolution in all morphological research, and impressed upon 

 modern morphology its characteristic stamp. The theory shows 

 that all the varied forms of organisms stand in genetic relation- 

 ship to one another by descent, and that ultimately all have been 

 derived from the simplest organisms. The theory of selection 

 ascribes the enormous variety of forms to natural selection con- 

 ditioned by the struggle for existence ; in this struggle only those 

 individuals of a generation survive that are best adapted to 

 existing external conditions in other words, those that are best 

 fitted to live. Thus, after an oblivion of more than two thousand 

 years the ancient idea of Empedocles of the descent and gradual 

 change of the organic world by selection has celebrated its 

 resurrection in the present century by the empirical foundation - 

 work of Darwin. Embryology, so far as it relates to the develop- 

 ment of form in organisms, has flourished to an unexpected degree 

 from the powerful stimulus given it as the result of Darwin's 

 theory, especially by Haeckel and his pupils, but so far physiology 

 has not availed itself of the evolution idea. The evolution of 

 vital activities, the origin and development of the many functions 

 possessed by the individual parts of the living body, is thus far 

 almost a terra incognita. During the last few decades but one 

 physiological problem of evolution, the problem of heredity, has 

 been very actively discussed, and this almost exclusively from the 

 zoological side. But the point has now been reached where 

 experimental physiology alone is able to bring about further 

 advance. 



III. THE METHOD OF PHYSIOLOGICAL RESEARCH 



It has been learned that the problem of physiology lies in the 

 explanation of vital phenomena, and it has been seen, in its 

 main features, how physiological research has developed in the 

 course of history. It is now incumbent upon us to summarise 

 with reference to the development of science what physiology has 



