INTRODUCTION 7 



of the sea. In the higher animals a greater stability in face of such 

 changes has been accomplished by the development of a heat-regulating 

 mechanism, so that, provided sufficient food is available, the tempera- 

 ture of the body is maintained at a constant level, which represents 

 the optimum for the discharge of the normal functions of the consti- 

 tuent parts of the body. The presence of food material in the environ- 

 ment of the living organism is a necessary condition for its continued 

 existence. In some cases, and this we must assume to be the primitive 

 condition, the food material must be of a given character and form a 

 constant constituent of the surrounding medium. In the higher forms 

 however, the development of the complex digestive system has enabled 

 the organism to utilise many different kinds of food, while the storage 

 of any excess of food as reserve material in the organism, either in 

 the form of fats or carbohydrates, provides for a constant supply of 

 food to the constituent cells of the body, even when it is quite wanting 

 in the environment. Since plants depend for their food in the first 

 place on the carbohydrates produced within the chlorophyll corpuscles 

 out of the atmospheric carbon dioxide by the energy of the sun's rays, 

 necessary conditions for their existence will be sunlight and the presence 

 of this gas in the surrounding atmosphere. 



One other necessary condition for the existence of life is the presence 

 of water. Although this substance cannot furnish any energy to the 

 complex molecules of which the living matter is composed, it is an 

 essential constituent of all living matter, and takes part in all the 

 changes which determine the transformations of matter and energy in 

 the organism. 



This short summary of the chief characteristics of living beings 

 would be incomplete without the mention of what is perhaps their 

 distinctive feature, namely, organisation. Although little marked in 

 the lowest members of the living kingdom, where we can detect only 

 a speck of structureless material containing a few granules, of which 

 one or more, in consequence of their reaction to stains, are distinguished 

 by the name of a nucleus, in the higher members this organisation 

 becomes more and more marked. The increased complexity of 

 organisation, which we often speak of as histological differentiation, 

 runs parallel with increasing range of power of adaptation, and 

 with increasing efficiency of adaptive reactions attained by the setting 

 apart of special structures (organs) for the performance of definite 

 functions. This parallelism between the development of function and 

 structure justifies us in the assumption generally, though often only 

 tacitly, accepted by physiologists, that the structure is the deter- 

 mining factor for the function. We might regard the histological 

 differentiation as representing merely a continuation of the increasing 

 molecular complexity, which We assumed must accompany and 



