COMPARATIVE PATHOLOGY. 755 



of an Invertebrate exhibit the power of migrating through 

 the walls of the blood vessels to the seat of irritation or 

 injury ; in Vertebrates this power is common. 



Among Vertebrates, as the circulatory system becomes 

 gradually more highly developed from Tunicates onwards, 

 the number of extra-vascular phagocytes is reduced, and 

 more and more devolves upon those of the blood. In the fin 

 of a young newt an injury or an infection may be dealt with 

 solely by the migratory phagocytes of the connective tissue ; 

 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 Amoebae, 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 many 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. 



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 



