REPAIR IN EVOLUTION 87 



pathological accident in some ancestors of the mammalians. 

 However complex the embryology of the uterus and its 

 appendages, the broad facts are compatible with this 

 view, which is strengthened by the later parasitic history 

 of the offspring after birth. The mammae appear to be a 

 compromise between the needs of the infant and the pro- 

 tection of the mother ; it has been suggested that they 

 originated in sore or tender spots on the epithelium most 

 exposed to the assaults of the parasite. Whether this is 

 true or not, the growth of the nipple is a complex variation 

 depending on the mechanical action of sucking with a 

 reaction proliferation of the epithelial elements of the 

 sweat and sebaceous glands, and an increased blood-supply 

 as special maternal protection against oral infection. It 

 seems to me that few stronger instances can be found of 

 the fact that the development of many organs, if not all 

 of them, is the result of direct reactions or adaptations, 

 which are in the nature of repair to tissues otherwise likely 

 to suffer disastrously. 



It is large macroscopic results of this order which enable 

 us to reason about other finer reactions, and even help us 

 to link to the general process those of a microscopic and 

 ultra-microscopic character which we class under ' im- 

 munity." Such phenomena are reactions under stress 

 which, by the provocation of catalysts, influence life. If, 

 indeed, much of human character is similar reaction, 

 perfect or imperfect, to the infections to which the race has 

 been and still is exposed, psychology itself must at last be 

 classed as the result of physical reactions — a conclusion 

 fully in accord with the work of Pavlov on conditioned 

 reflexes. 



If any further illustration of the conclusions so far 

 suggested is necessary, it may be found in the fixation of 



