18 INTRODUCTION. 



manifestations can be reduced to simple physical laws. This state- 

 ment, however, is true only in part, for although the forces of which 

 we have cognizance as being at work in living bodies are the same 

 as those with which we are familiar in the non-organized world, 

 we are nevertheless unable to explain the phenomena of life upon 

 this basis, and are as yet forced to accept the existence of a vital 

 prineiple, of the nature of which we know nothing. 



Character of Chemical Changes. The chemical processes 

 which are involved in the transformation of non-living matter into 

 living tissue are qualitatively the same in plants and animals. 

 Quantitative differences, however, exist, which are sufficiently pro- 

 nounced to serve as marks of distinction between animals and 

 plants. Thus plants are capable of evolving from relatively simple 

 compounds those complex chemical substances which go to form 

 their structure, while animals apparently do not possess this power. 

 They are hence dependent for their existence upon food-stuffs which 

 are preformed ; and the potential energy which animals require 

 for the functioning of their various organs, and which they trans- 

 form into kinetic energy, is, as a matter of fact, derived in every 

 instance, either directly or indirectly, from plant-life. Plants, in 

 turn, obtain the potential energy which is stored in their tissues from 

 the kinetic energy of sunlight, and in virtue of this energy can 

 elaborate those simple chemical substances which are at their dis- 

 posal as food-stuffs into the complex bodies which constitute their 

 tissues. 



We thus observe that while in plant-life synthetic chemical 

 processes prevail, analytical processes are foremost in animal life. 

 These analytical processes, moreover, are largely of the character of 

 oxidations, while the syntheses which are effected in the bodies of 

 plants are essentially of the nature of reductions. But just as syn- 

 thetic processes are not absolutely characteristic of plant-life, so also 

 do oxidation-processes occur in plants, and synthetic reductions in 

 animals. This becomes especially noticeable as we descend in the 

 scale of both animal and vegetable life. Primitive vegetable 

 organisms are thus met with which, like the highly organized mam- 

 mal, are almost entirely dependent for their existence upon already 

 elaborated food-stuffs, and low forms of animal life similarly occur 

 in which the processes of nutrition are essentially the same as those 

 which occur in the higher plants. The differences which thus exist 

 between animal life and plant-life are therefore, as has been stated, 

 more of a quantitative than a qualitative kind. 



Synthetic Processes in Plants. I have said that plants are 

 capable of elaborating from simpler compounds the complex chem- 

 ical substances of which they are composed, and that the chemical 

 processes here involved are essentially of the nature of synthetic 

 reductions. Formerly, it was believed that the various organic sub- 

 stances which occur in animals and plants could be produced only 

 through the agency of a special vital force ; but we now know that 



