, effected in rnat^y cases within a fraction of a second. But in the evolution 

 of life upon this earth, this method of adaptation is of comparatively late 

 appearance and is" confined almost entirely to one division of living beings 

 i.e., the animal kingdom. In the lowest organisms, the unicellular, 

 such as the bacteria and protozoa, the only adaptations, into the mechanism 

 of which we can gain any clear insight, are those to the environment of 

 the organism, and in these cases the mechanism is almost entirely a 

 chemical one. The organism approaches its food or flies from harmful 

 media in consequence of chemical stimuli ; it prepares its food for 

 digestion or digests it by the formation of chemical substances, toxins or 

 enzymes. In the lowest metazoa, such as the sponges, there is still no 

 trace of any nervous system. The coordination between the different 

 cells of the colony is still determined by purely chemical means. The 

 aggregation of the phagocytic cells round a foreign body is apparently 

 due to the attraction exerted on them by the chemical substances 

 produced in the death of the injured tissues. 



With the appearance of a central nervous system or systems in the 

 higher metazoa, the quick motor reactions determined by this system 

 form the most obvious vital manifestations of the animal. But the 

 nervous system has been evolved for quick adaptations, not for the 

 abolition of the chemical correlations which existed before a nervous 

 system came into being. A study of the phenomena of even the highest 

 animals shows that the development of the quick nervous adaptations 

 involves no abrogation of the other more primitive class of reactions i.e., 

 the chemical ones. Where the reaction is one occupying seconds or 

 fractions of a second the nervous system is of necessity employed. Where 

 the reaction may take minutes, hours, or even days for its accomplishment, 

 the nexus between the organs implicated may be chemical. Already we 

 are able, in many cases, to prove the existence of such a chemical nexus, 

 and to employ it in artificially producing a state of growth or activity, 

 which is in normal circumstances merely a phase in a complex series of 

 physiological changes. 



The chemical reactions or adaptations of the body, like those which 

 are carried out through the intermediation of the central nervous system, 

 can be divided into two main classes (i) those which are evoked in 

 consequence of changes impressed upon the organism as a whole from 

 without ; and (2) those which, acting entirely within the body, serve to 

 correlate the activities, in the widest sense of the term, of the different 

 parts and organs of the body. 



The first class of adaptations includes the reactions of the body to 

 chemical poisons produced by bacteria or higher organisms, and repre- 

 sents one of the most important means by which the body maintains 

 itself in the struggle for existence. The complicated phenomena involved 

 in the formation of antitoxins, of cytolysins, of bactericidal substances, 

 and such like means of protection, have been the subject of much study 

 of recent years, and their immediate interest to the practical physician 

 it unnecessary for me to devote any time to their discussion, 



