334 ANIMAL NATURE AS A WHOLE. [in. 



is the perfect or adult stage of life. In some animals it is ex- 

 tremely short, in others prolonged; but without it no animal 

 is complete, and so soon as it terminates, the animal begins to 

 decline. This period is also the great object of natiu-e in the 

 animal creation, and to this there is no exception from man to 

 the simplest animalcule. 



653. In sentient animals the propagation of the species 

 takes place by means of the satisfaction of the instinct for 

 sexual congress (289) : in insentient animals which propagate 

 by copulation, this instinct is replaced by nerve-actions. It has 

 been already illustrated fully (274, 289, 290, 481, 540, 560) ; 

 and we have only to consider here the changes which take 

 place at this period in animal nature. 



654. All animals are not bom with sexual organs : some 

 only acquire them after undergoing a succession of transforma- 

 tions, as insects. But these organs in those animals which 

 possess them at birth, as well as the body generally, undergo 

 such great changes, as the period of propagation of the species 

 approaches, that the period itself becomes of the greatest and 

 most general importance. The whole body is invigorated, the 

 seminal fluid is secreted, and often communicates its odour to 

 the whole animal ; smooth parts become hairy, horns grow, the 

 voice alters, &c. In the female equally important changes 

 take place. 



655. It is a necessary result, that great modifications of 

 the nervous system accompany these important changes in the 

 animal. The nerves of the whole body, and particularly of the 

 sexual organs, become susceptible of new impressions, and in 

 sentient animals this occurs so extensively, that it appears as 

 if a wholly new sense of feeling had been developed. A look, 

 a tone, an odour, a touch, which the sentient animal had ex- 

 perienced a thousand times before, without any other than 

 ordinary results, cause an emotion during this period, which 

 excites the instinct to sexual congress, while the sexual organs 

 themselves undergo analogous changes (274, 289). The external 

 impressions of these sensations in sentient animals excite the 

 same unusual operations in the insentient (481). Without 

 exciting a sensation, they are so reflected as to act upon the 

 sexual organs, as if they had excited a sensation (540). That 

 external impressions are so reflected on the nerves of the sexual 



