io COMPARATIVE ANATOMY AND PHYSIOLOGY. 



(1) Organs of internal relations. 



i. Protective. Examples : Skin, shell. 



ii. Nutritive. Examples : Digestive tract (nu- 

 trient) ; heart and blood-vessels (circulatory). 



iii. Purifying 1 . Gills, lungs (carbonic acid) ; kid- 

 neys (nitrogenous products). 



(2) Organs of external relations. 



iv. LiOCOmotor. Limbs, etc. (compounded of 



skeletal and muscular tissues). 

 v. Prehensile. Limbs, etc. (compounded of 



skeletal and muscular tissues). 

 vi. Offensive. Teeth, claws, electrical, odorous 



organs. 



(3) Reproductive. 



(a) Germ -producing glands : testes, ovaries, which are 



essential. 

 (/3) Copulatory : penes, etc., which are accessory. 



(4) Sensory. 



(a) Organs which receive impressions ; eye, ear, 



etc., brain. 

 (j8) Organs which Stimulate other organs ; brain. 



METHODS OF COMPARISON. 



When an anatomist has acquired a positive 

 knowledge of a certain number of selected forms of 

 life, he proceeds to convert his empirical acquaintance 

 with facts into science by reasoning upon the infor- 

 mation which he has acquired. In this operation he 

 makes great use of the fertile method of comparison, 

 remembering the words of Buflbn, " Ce riest qu'en 

 comparant que nous pouvons juger." Like things, 

 however, must be compared with like, or confusion 

 will inevitably result. We must, therefore, lay down 

 certain rules to guide us in these kinds of enquiries, 

 for, though no one would attempt to compare a heart 

 with a lung, many would, at first, more willingly 

 compare the leg of a man with that of a cockroach, 

 than the fin of a perch with the wing of the sparrow ; 

 yet the latter is the more justifiable proceeding. 



