20L MH. MORLEY ROBERTS OX THE FrXCTlOX OF 



has been and still is expose*! Thus psTcliologv itself must at 

 last be classed as the result of physical reactions, a conclu- 

 sion fully in accord with the work of Pavloff on conditioned 

 reflexes. 



If any further illustration of the conclusions so far suggested 

 is necessaiy. it may be found in the growth, of the mesentery. 

 It has often been pointed out that the embryonic processes bv 

 which it is formed are histologically those of plastic organized 

 exudations. The attachments of the whole tube do not come 

 about at the same period of foetal development, and it seems of 

 significanc-e when we note that the mesentery of the small gut has 

 an oblique attachment, to the posterior abdominal wall from the 

 duodenum to the right iliac fossa, only found in animals which 

 have assumed the upright posture. This comes into existence as 

 late as the foui-th or fifth month of fcetal development. Before 

 this band was formed there must have been a great series of 

 disasters, for even now the last part of the mesentei-y to become 

 attached to the abdominal wall, that is, the angle between the 

 ileum and ascending colon, sometimes remains free. A volvulus 

 may easily form there by rotation of the ileo-colic loop. The 

 whole history suggests a series of lymph effusions, caused bv 

 pathological states, some of which were soi-ted out by the lethal 

 process of natural selection, the remainder surviving and leaving 

 offspring with the liability to organize the effusion in the safe 

 way. The pathology of those cases in which what are known as 

 Lane's Xinks can be found is obviously of a similar character. 

 The stasis of the affected bowel causes lymph effusion and the 

 formation of a band which is morphologic-ally homologous with 

 the early mesentery. 



After reviewing phenomena such as these, the conclusion 

 seems inevitable that single small favoiu^ble variations have 

 not done the whole work of evolution. They may play their 

 part as coiTelated changes, but they then take their place in a 

 series of which the causes can be recognized. In combination 

 with reasonable views of nse and disuse and of increased or 

 decreased blood-supply they may, perhaps, be held to explain 

 such phenomena as the delicate co-aptation of some c-ardiac 

 valves. Their place in the explanation of the phenomena of 

 mimiciy seems obvious. But though they may help us to com- 

 prehend how tissues become finished structures if they are 

 combined with the results of functional energy, they yield no 

 hint as to great or decisive developments and the mechanism 

 involved in them. If the reasons adduced for the thesis laid 

 down carry any weight, it is obvious that many, if not most, of 

 the really decisive variations in all internal structure depended 

 and still depend, rot on variations which can be called favourable 

 but on those that for the major portion of the organisms involved 

 are directly disastrous : not on variations which are small but 

 on those which are big enough to be appreciable as the cause of 



