INTRODUCTION. 57 



they are different in their conditions and in their results, and are 

 consequently peculiar and characteristic. 



Another set of vital phenomena are those which are manifested 

 in the processes of reproduction and development. They are again 

 entirely distinct from any phenomena which are exhibited by 

 matter not endowed with life. An inorganic substance, even when 

 it has a definite form, as, for example, a crystal of fluor spar, has 

 no particular relation to any similar form which has preceded, or 

 any other which is to follow it. On the other hand, every animal 

 and every vegetable owes its origin to preceding animals or vege- 

 tables of the same kind ; and the manner in which this production 

 takes place, and the different forms through which the new body 

 successively passes in the course of its development, constitute the 

 phenomena of reproduction. These phenomena are mostly depend- 

 ent on the chemical processes of nutrition and growth, which take 

 place in a particular direction and in a particular manner ; but their 

 results, viz., the production of a connected series of different forms, 

 constitute a separate class of phenomena, which cannot be explained 

 in any manner by the preceding, and require, therefore, to be studied 

 by themselves. 



Another set of vital phenomena are those which belong to the 

 nervous system. These, like the processes of reproduction and 

 development, depend on the chemical changes of nutrition and 

 growth. That is to say, if the nutritive processes did not go on in 

 a healthy manner, and maintain the nervous system in a healthy 

 condition, the peculiar phenomena which are characteristic of it 

 could not take place. The nutritive processes are necessary condi- 

 tions of the nervous phenomena. But there is no other connection 

 between them ; and the nervous phenomena themselves are distinct 

 from all others, both in their nature and in the mode in which they 

 are to be studied. 



A troublesome confusion might arise if we were to neglect the 

 distinction that really exists between these different sets of phe- 

 nomena, and confound them together under the expectation of 

 thereby simplifying our studies. Since this can only be done by 

 overlooking real points of difference, its effect will merely be to 

 introduce erroneous ideas and suggest unfounded similarities, and 

 will therefore inevitably retard our progress instead of advancing it. 



It has been sometimes maintained, for example, that all the vital 

 phenomena, those of the nervous system included, are to be reduced 

 to the chemical changes of nutrition, and that these again are to be 



