328 ANIMAL NATURE AS A WHOLE. [iii. 



631. The organs of generation are very different in different 

 animals. In the higher classes in which there are two sexes 

 the instinct of propagation of species is requisite, its satisfac- 

 tion (or copulation) having no other natural object than the 

 fertilization of the ovum. The objects aimed at in the various 

 modes of generation are concealed from us, but in animals 

 endowed with mind, the intent of the difference of the two 

 sexes seems to be this, that the sexes may be led to associate 

 in protecting their offspring, and providing for their various 

 wants. 



633. So far as our observations on the development of the 

 ovum extend, we find that the new animal is a portion sepa- 

 rated from the parent, duly furnished with both animal and 

 mechanical machines necessary to its existence and develop- 

 ment. With regard to those animals which produce their 

 young like themselves, we find that their offspring is furnished 

 with all the essential animal structures, so soon as separated 

 from the parent. 



633. The whole process of generation is a masterpiece of 

 nature, and animal bodies are constructed most skilfully to this 

 end, so that even the most intelligent animals do not possess 

 the least knowledge of the varied arrangements and plans of 

 nature in carrying it on (289, 290). This is not the place, 

 however, for details. 



634. The period of development comprised between the 

 time of conception and of birth, is a period of imperfection, 

 and, consequently, no animal can be expected to have the full 

 use of its natural animal forces ; it is for this reason that insen- 

 tient animals and even sections of polypes are inactive during 

 this period, until full development be attained. In the more 

 perfect animals it is different after the appearance of the 

 pundum saliens, for this movement is a direct nerve-action of 

 the external impression of the blood on the heart (459), and, 

 consequently, sentient animals have at least the use of the vis 

 nervosa to some extent, and, therefore, possess the nature of 

 living organisms. The intestines of the chick in ovo are not 

 irritable during the first fourteen days, but subsequently they 

 become more and more irritable every day. (Haller^s ' Opera 

 Minora,' p. 401, tom. ii, pp. 364, 398.) As development ad- 

 vances, (in man about the fourth month,) the foetus is excited 



