308 HAND-BOOK OF PHYSIOLOGY. 



the lowest, and in some plants, the portions of organized structure special- 

 ized for development after their mutual union into a new individual, are 

 found on two distinct beings, which we call respectively male and female. 



The old idea concerning the power of growth resident in the germ of 

 the new being, thus formed in various ways, was expressed by saying that 

 a store of dormant vitality was laid up in it, and that so long as no de- 

 composition ensued, this was capable of manifesting itself and becoming 

 active under the influence of certain external conditions. Thus, the 

 dormant force supposed to be present in the seed or the egg was assumed 

 to be the primary agent in effecting development and growth, and to con- 

 tinue in action daring the whole term of life of the living being, animal 

 or vegetable, in which it was said to reside. The influence of external 

 forces heat, light, and others was noticed and appreciated; but these 

 were thought to have no other connection with vital force than that in 

 some way or other they called it into action, and that to some extent it 

 was dependent on them for its continuance. They were not supposed to 

 be correlated with it in any other sense than this. 



Now, however, we are obliged to modify considerably our notions and 

 with them our terms of expression, when describing the origin and birth 

 of a new being. 



To take, as before, the simplest case a seed or egg. We must sup- 

 pose that the heat, which in conjunction with moisture is necessary for 

 the development of those changes which issue in the growth of a new 

 plant or animal, is not simply an agent which so stimulates the dormant 

 vitality in the seed or egg as to make it cause growth, but it is a force, 

 which is itself transformed into chemical and vital power. The embryo 

 in the seed or egg is a part which can transform heat into vital force, this 

 term being a convenient one wherewith to express the power which par- 

 ticular structures possess of growing, developing, and performing other 

 actions which we call vital. 1 Of course the embryo can grow only by 

 taking up fresh material and incorporating it with its own structure, 

 and therefore, it is surrounded in the seed or ovum with matter sufficient 

 for nutrition until it can obtain fresh supplies from without. The ab- 

 sorption of this nutrient matter involves an expenditure of force of some 

 kind or other, inasmuch as it implies the raising of simple to more com- 

 plicated forms. Hence the necessity for heat or some other power before 

 the embryo can exhibit any sign of life. It would be quite as impossible 

 for the germ to begin life without external force as without a supply of 

 nutrient matter. Without the force wherewith to take it, the matter 

 would be useless. The heat, therefore, which in conjunction with mois- 



1 The term " vital force " is here employed for the sake of brevity. Whether it is 

 strictly admissible will be discussed hereafter. 



The general term force is used as synonymous with what is now often termed 

 energy. 



