OF REPRODUCTION. 585 



or chiefly upon the temperature of the surrounding 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, pro- 

 vided its temperature be high. We learn from these facts the great import- 

 ance of not suddenly checking Exhalation, 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 sus- 

 pended. 



CHAPTER XIV. 



OF REPRODUCTION. 



I. General character of the Function. 



732. 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 

 origin 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, 

 may be employed in generating new individuals ; since, when the body is 

 divided into numerous parts, each one of these has the power of developing 

 all the rest of the structure, and thus of becoming a complete animal ( 21). 



* See Principles of General and Comparative Physiology, 98. 



