2 PHYSIOLOGY 



But if 'Biology is to include the complete study of life in all 

 its manifestations, it represents a field too vast to admit of 

 comprehension by any single mind in all its details. Hence the 

 necessity arises for a further division of labour. 



I. If at any given moment of its existence we set out to 

 consider the mode of life and action of any living being, we can at 

 once distinguish the morphological characteristics, depending on 

 anatomical and histological structure, from the functional or 

 physiological features, which are dependent on its cytological, 

 physical, and chemical constitution. If in living beings we 

 consider the development, the perpetual becoming, in other words 

 the morphological and physiological changes they undergo from 

 beginning to end of their existence, we have the story of 

 Evolution, which enables us to a certain point, both for the indi- 

 vidual and the species, to follow the different phases of development 

 as these fulfil themselves according to the great laws of heredity 

 and variation. 



The complete study of life, to which the term Biology has thus 

 been applied, is appropriately divided into three branches : 



(a) Morphology, which covers the forms of living beings, i.e. 

 the cellular elements from which the tissues are built up, the 

 connections of the tissues whence the organs develop, the 

 structure of the organs and systems. 



(6) Physiology, which covers the functions or activities of 

 living beings, and the various cytological, physical, and chemical 

 factors from which these arise : in other words, the storage and 

 dispersal of the energies of which organisms are the seat, and the 

 phenomena or external manifestations by which they are revealed 

 to us. 



(c) Biogenesis, i.e. the story of evolution, morphological as well 

 as functional, whether ontogenetic, for the individual, or phylo- 

 genetic, for the race. 



The intimate connections of these three great branches of 

 biological science are obvious. Since organic form is the necessary 

 matrix of function, the study of Physiology perforce includes that 

 of Morphology, or Anatomy as the latter is commonly but loosely 

 termed. These two branches are really offshoots of the same 

 trunk, inasmuch as they constituted in bygone times a single 

 science professed by a single teacher, when the (vastly pre- 

 dominating) study of the morphological signs of life was identified 

 in various ways with that of its physiological properties. But as 

 the study of form has methods of research and problems which are 

 separate and quite distinct from those relating to function, 

 anatomy has gradually detached itself from physiology, pursuing 

 its own independent development. The History of Evolution or 

 Biogenesis, again, which covers a vast field of researches in em- 

 bryology, comparative anatomy, and palaeontology, is evidently 



