COMPARATIVE PATHOLOGY. 751 



in the most frequently observed case— the tail of a tadpole 

 in which the blood vessels are formed — the extra-vascular 

 phagocytes are greatly aided by leucocytes, which work their 

 way through the walls of the vessels or are liberated by a 

 lesion ; in other cases all may depend on these leucocytes. 

 It is important also to notice that the endothelial cells of 

 the blood vessels seem by their contractility to assist the 

 passage (or diapedesis) of the leucocytes ; sometimes, more- 

 over, they may themselves leave the wall of the vessel to 

 deal with Bacteria introduced into the blood. 



We are not here concerned with Metchnikoff's thesis that 

 " inflammation generally must be regarded as a phagocytic 

 reaction on the part of the organism against irritants — a 

 reaction carried out by the mobile phagocytes sometimes 

 alone, sometimes with the aid of the vascular phagocytes or 

 of the nervous system." We are immediately interested 

 only in noticing how these mobile cells, retaining many of 

 the qualities of the ancestral Amcebfe, perform in the animal 

 body numerous functions, struggling with invading Bacteria, 

 surrounding and engulfing irritant particles, and repairing 

 wounds. And from the most general point of view it is 

 evident that one of the numerous factors determining the 

 fate of an organism in the struggle for existence is its power 

 of resisting Bacteria. If phagocytes be not present, there 

 must be some other means of defence ; thus the Nematodes 

 have found this in their firm resistent cuticles. 



The processes of disease in higher animals have been very 

 carefully investigated from the evolutionist's point of view 

 by Sutton. He points out that some of the causes which 

 pathologists recognise as operating to produce disease, 

 (viz., hypertrophy or atrophy of organs or structures, and 

 coalescence of parts originally distinct), are also "factors in 

 evolution," which biologists recognise in their theories of 

 the progress of life. Thus, descending to particular cases, we 

 find that the long claws of the sloth and bat, the great 

 curved teeth of the Babirussa, are paralleled in pathological 

 conditions by the elongated nails and hoofs of Birds and 

 Ungulates kept in unnatural conditions, by the curved 

 incisors of Rodents which have lost the corresponding teeth 

 of the other jaw. It is unnecessary here to multiply examples 

 of greatly hypertrophied organs, normally present in certain 



