14 LIFE. VITAL STIMULI. [iNTROD. 



chemical affinity, each has its share in promoting the functions in 

 the play of which Life consists ; all are more or less necessary to 

 the integrity of these actions ; and it is contrary to experience to 

 suppose that Life can he manifested without their co-operation. 

 Yet we cannot say that Life is produced by, or is the result of, 

 these agencies. It is equally contrary to experience to find the 

 manifestation of Life in other than organized bodies. Nevertheless, 

 it cannot be affirmed that organization is the cause of Life, for 

 without the other agents no vital action occurs, and it has already 

 been shewn that the organization of new matter is effected only by 

 living bodies. The mutual co-operation of organized matter with 

 the forces at work in the inorganic world, is necessary to the de- 

 velopment of vital phenomena. 



The term Life, then, may be regarded as denoting an ultimate 

 fact in science, which may be thus expressed ; that certain com- 

 pounds of matter which, as being artfully arranged in a particular 

 form for a special end, and associated together by a certain me- 

 chanism, are called organized do, by their co-operation with phy- 

 sical and chemical forces, manifest a train of phenomena, which are 

 of the same, or of an analogous kind, for all organized beings ; that 

 is to say, they manifest the phenomena of Life. All organised 

 substances, capable of thus co-operating with the other natural 

 agencies, are called living ; and, although they may not be posi- 

 tively in action, they are yet alive, as being ready to act when the 

 complementary conditions to vital action shall be supplied to them. 

 Thus the seed is alive, although not in action ; but, immediately 

 it is brought into contact with moisture and heat, life is manifested. 

 Hence these agents, moisture, heat, light, &c. are said to act as 

 vital stimuli. The organic matter, in becoming part of a living 

 machine, acquires certain properties, very different from what it 

 possessed before ; these are called vital properties : they continue 

 as long as the organization remains unchanged. For example, a 

 certain proximate element is organized to form muscle ; it then 

 acquires the property of contractility, which it retains during life. 



According to our experience, organic matter derives vital pro- 

 perties in by far the majority of instances, and probably in all, from 

 a previously existing organism. The egg, while within the body of 

 the mother, acquires vital properties ; and it manifests an inde- 

 pendent life when it is laid, if the requisite conditions (vital stimuli) 

 are then supplied. Thus is life transmitted from one living being 

 to another; and the life of a present generation of animals and 

 plants has its source in that of a previous generation. If we trace 



