G THE HFXilNMNCiS OF COMPARATIVE ANATOMY 



sponges, was grouped with the Testacea. The first five 

 groups were classed together as sanguineous, the others as 

 exsanguineous, from the presence or absence of red blood. 



Besides these classes " there are," he says, " many other 

 creatures in the sea which it is not possible to arrange in any 

 class from their scarcity " (Creswell, loc. cit., p. 90). 



(3) Aristotle's greatest service to morphology is his clear 

 recognition of the unity of plan holding throughout each of 

 the great groups. 



He recognises this most clearly in the case of man and 

 the viviparous quadrupeds, with whose structure he was best 

 acquainted. In the Historia Aniuialimn he takes man as a 

 standard, and describes his external and internal parts in 

 detail, then considers viviparous quadrupeds and compares 

 them with man. " Whatever parts a man has before, a 

 quadruped has beneath ; those that are behind in man form 

 the quadruped's back" (Cresswell, loc. cit., p. 26). Apes, 

 monkeys, and Cynocephali combine the characteristics of man 

 and quadrupeds. He notices that all viviparous quadrupeds 

 have hair. Oviparous quadrupeds resemble the viviparous, 

 but they lack some organs, such as ears with an external 

 pinna, mammae, hair. Oviparous bipeds, or birds, also "have 

 many parts like the animals described above." He does not, 

 however, seem to realise that a bird's wings are the equivalent 

 of a mammal's arms or fore-legs. Fishes are much more 

 divergent ; they possess no neck, nor limbs, nor testicles 

 (meaning a solid ovoid body such as the testis in mammals), 

 nor mammas. Instead of hair they have scales. 



Speaking generally, the Sanguinea differ from man and 

 from one another in their parts, which may be present 

 or absent, or exhibit differences in "excess and defect," 

 or in form. Unity of plan extends to all the principal 

 systems of organs. "All sanguineous animals have either a 

 bony or a spinous column. The remainder of the bones 

 exist in some animals; but not in others, for if they have 

 the limbs they have the bones belonging to them" (Cress- 

 well, loc. cit., p. 60). " Viviparous animals with blood and 

 feet do not differ much in their bones, but rather by analogy, 

 in hardness, softness, and size" (Cresswell, loc. cit., p. 59). 



