cH. IV.] ANIMAL LIFE. 333 



^ 



648. In fact, we see that all animals perform more numerous 

 and perfect acts as they approach the period of perfect de- 

 velopment. Many animals, especially the insentient, act from 



e beginning with great skill and adaptiveness, but they can- 

 t perform those movements which belong to a more perfect 

 stage. A caterpillar acts principally from the instinct for nutri- 

 tion ; it must undergo several changes, before it is capable of the 

 animal act of spinning : it is only when fully developed into a 

 butterfly, that it can perform the acts necessary to propagation 

 of the species. No signs of a desire to perform motions of 

 this kind are manifested before the organs requisite to their 

 performance are developed. (Compare Spallanzani, ' Phys. 

 Abhand.,^ p. 167, for examples in the infusoria.) 



649. The brain is always imperfect at first in sentient animals. 

 During the period of growth, it becomes larger and firmer, and 

 receives a movement from the respiratory act, which it had not 

 before, and which seems to have an influence on the actual use 

 of the cerebral forces. The nerves also increase in sentient 

 animals after birth, not less than in the insentient, and parts 

 become sensitive and irritable which were not so formerly. 



650. In sentient animals, as w ell as in insentient, we observe 

 a progressive development of the animal forces ; although from 

 the moment of birth many of these display a readiness in the 

 use of the vis nervosa, and a perfection in the instincts necessary 

 to their maintenance, so as greatly to surpass man in these 

 respects. Still many of their sensational and motor faculties 

 are so imperfect, that the earliest portion of their existence is 

 only the vague dream of an almost continuous sleep. 



651. Since it is during the period of growth that the nervous 

 system and its forces are developed, it is at that time that they 

 determine the temperament of the animal (52, 502), together 

 with its animal and even moral character, if capable of such 

 (65, 295, 344). Now, as every genus and species of animal 

 has originally the capabilities of its parents, and these only be- 

 come more perfect and fixed during the period of growth, it 

 follows that every genus and species have each their special 

 characteristics. These capabilities and endowments may be 

 changed in numerous ways, by habit, education, and accidental 

 modifications of development (52, 431, 501). 



652. The third period, or that of propagation of the species. 



