20 INTRODUCTION. 



it is nourished as if it were one of its organs ; and when it detaches 

 itself, it possesses its own life, which is essentially similar to that of 

 the adult. 



The germ, the embryo, the foetus, and the new-born animal, have 

 never, however, exactly the same form as the adult, and the difference 

 is sometimes so great that their assimilation has been termed a meta- 

 morphosix. Thus, no one not previously aware of the fact would 

 suppose that the caterpillar is to become a butterfly. 



Every living being is more or less metamorphosed in the course of 

 its growth ; that is, it loses certain parts, and developes others. The 

 antennae, wings, and all the parts of the butterfly, were enclosed beneath 

 the skin of the caterpillar ; this skin vanishes along with the jaws, feet, 

 and other organs, that do not remain with the butterfly. The feet of 

 the frog are enclosed by the skin of the tadpole ; and the tadpole, to 

 become a frog, parts with hs tail, mouth, and branchiae. The child, at 

 birth, loses its placenta and membranes ; at a certain period its thymus 

 gland nearly disappears, and it gradually acquires hair, teeth, and 

 beard ; the relative size of its organs is altered, and its body augments 

 in a greater ratio than its head, the head more than the internal 

 ear, &c. 



The place where these germs are found, and their germs themselves, 

 are collectively styled the ovary; the canal through which, when 

 detached, they are carried into the uterus, the oviduct ; the cavity in 

 Avhich, in many species, they are compelled to remain for a longer or 

 shorter period previous to birth, the uterus. 



OP THE INTELLECTUAL FUNCTIONS OP ANIMALS. 



The impression of external objects upon the individual, the produc- 

 tion of a sensation or of an image, is a mystery into which the human 

 understanding cannot penetrate ; and materialism is an hypothesis, 

 so conjectural, that philosophy can furnish no direct proof of the 

 actual existence of matter. The naturalist, however, should examine 

 what appears to be the material condition of sensation, trace the 

 ulterior operations of the mind, ascertain to what point they reach 

 in each being, and assure himself whether they are not subject to 

 conditions of perfection, dependent on the organisation of each species, 

 or on the momentary state of each individual body. 



To enable a being to perceive, there must be an uninterrupted 

 communication between the external sense and the central masses of 

 the medullary system. It is, then, only the modification experienced by 



