30 NATURE AND OBJECTS OP PHYSIOLOGICAL SCIENCE. 



Physics, we cannot ascend above the fact of Attraction (which operates 

 according to a simple and universal law) between all masses of matter ; 

 and in Chemistry, we cannot rise beyond the fact of Affinity (limited by 

 certain conditions which are not yet well understood) between the par- 

 ticles of different kinds of matter. When we say that we have explained 

 any phenomenon, we merely imply that we have traced its origin to 

 properties with which we were previously acquainted, and shown that it 

 takes place in accordance with the known laws of their operation. Of 

 the existence of the properties, and the determination of the conditions 

 of their action, we can give no other account, than that the Creator 

 willed them so to be ; and, in looking at the vast variety of phenomena 

 to which they give rise, we cannot avoid being struck with the general 

 harmony that exists amongst them, and the mutual dependence and 

 adaptation that may be traced between them, when they are considered 

 as portions of the general economy of Nature. There is no difference 

 in this respect between Physiology and other sciences ; except that the 

 number of these (apparently) ultimate facts is at present greater in 

 physiology than it is in other departments, because we are not at present 

 able to include them all under any more general expression. But, as 

 will presently appear, a considerable degree of simplification appears 

 practicable in our view of them ; and although we may not be able to 

 say why the structure called Muscular should possess contractility, and 

 why the structure called Nervous should be capable of generating and 

 conveying the force which excites that contractility to action, we may 

 draw, from the study of the conditions under which they respectively 

 manifest themselves, some indications of the existence of a common tie, 

 such as that which binds together the planetary masses, at the same 

 time that it weighs down the bodies on the surface of the earth towards 

 its centre. 



26. In the study of any branch of science, it is most desirable to com- 

 mence with definite views of the nature of the phenomena with which it 

 is concerned ; and such are best gained by the examination of these 

 phenomena under their simplest aspects. This course is most especially 

 necessary in Physiology : since the complexity of the conditions under 

 which its phenomena usually present themselves, often tends to mask 

 their real character, causing that to be regarded as essential which is 

 only accidental or contingent, and vice versd. It is extremely difficult, 

 however, and frequently impossible, for the Physiologist to isolate these 

 several conditions, and to study them separately, in the way that the 

 Chemical or Physical investigator would do ; and his best course is to 

 take advantage of those "experiments ready prepared by Nature," 

 which he finds in the variety of forms of living organized beings, with 

 which the globe is so richly peopled. Now it is in the simplest forms of 

 Cryptogamic Vegetation, that the phenomena of Life present themselves 

 under their least complicated aspect ; for we shall find in the operations 

 of each of the simple cells of which such Plants are composed (all of 

 them resembling one another in structure and actions), an epitome, as it 

 were, of those of the highest and most complex Plant ; whilst those of 

 the higher Plants bear a close correspondence with those which are 

 immediately concerned in the Nutrition and Reproduction of the Animal 



