OF REPRODUCTION. 687 



surface of the body in this manner. In cold weather, very little is thus carried 

 oft", even though the air be dry : and a warm atmosphere, already charged 

 with dampness, will be nearly as ineffectual. But simple evaporation is not 

 the chief means by which the temperature of the body is regulated. The 

 Skin, as already mentioned ( 868), contains a large number of glandular, the 

 office of which is to secrete an aqueous fluid ; and the amount of this Exha- 

 lation appears to depend solely or chiefly upon the temperature of the sur- 

 rounding air. Thus, when the external heat is very great, a considerable 

 amount of fluid is transuded from the skin ; and this, in evaporating, converts 

 into latent heat a large quantity of the free caloric, which would otherwise 

 raise the temperature of the body. If the atmosphere be hot and dry, and 

 also be in motion, both Exhalation and Evaporation go on with great rapidity. 

 If it be cold, both are checked, the former almost entirely so ; but if it be 

 dry, some evaporation still continues. On the other hand, in a hot atmosphere, 

 saturated with moisture, Exhalation continues, though Evaporation is almost 

 entirely checked; and the fluid poured out by the exhalant glands accumulates 

 on the skin. There is reason to believe that the secretion continues, even 

 when the body is immersed in water, provided its temperature be high. We 

 learn from these facts the great importance of not suddenly checking Exhala- 

 tion, by exposure of the surface to cold, when the secretion is being actively 

 performed ; since a great disturbance of the circulation will be likely to ensue, 

 similar to that which has been already mentioned, as occurring when other 

 important secretions are suddenly suspended. 



CHAPTER XVII. 



OF REPRODUCTION. 



1. General Character of the Function. 



898. THE Function of Reproduction has been commonly regarded as so 

 entirely different in character from the ordinary Nutritive processes, that no 

 analogy can be drawn between them. The results of late inquiries, however, 

 leave no doubt that the difference between them is extremely small, having, 

 in fact, a relation rather to the object of the action, than to the mode in which 

 it is performed. In the ordinary function of Nutrition, there is a continual 

 regeneration or reproduction of the tissues and organs of the body; but the 

 new parts are destined still to constitute the same whole. On the other hand, 

 in Reproduction, the newly-formed parts are destined from the first to be cast 

 off from the parent structure, and to become new individuals. Still, their ori- 

 gin is essentially the same in both instances ; as appears from the mode in 

 which the multiplication of the lower Plants and Animals takes place. Thus 

 in the simplest Cryptogamia, such as the Yeast Fungus,* every single cell 

 may be regarded as a distinct individual ; since it is capable of living by itself, 

 and of generating new cells ; and thus the production of a new cell, in con- 

 nection with the original one, may be regarded as alike an act of Nutrition 

 and of Reproduction. So again, in the Hydra and other Polypes, the remark- 

 able power of reparation which is manifested in their Nutritive operations, 



* See Principles of General and Comparative Physiology, 98. 



