22 NATURAL SCIENCE [July 



put upon it, i.e., if the assumed active life was to be maintained. 

 Such a division of the body is the only one which affords not only 

 direct continuity, but also the closest possible association, between 

 the dorsal neuro-muscular region and the great ganglia of the sensory 

 organs at the anterior end of the body. That such a division of 

 labour actually existed between the dorsal and ventral halves of the 

 primitive vertebrate body, the former being the muscular and 

 locomotor region while the latter was the vegetative region, is shown 

 by the transverse section through the middle of the body of a low 

 vertebrate. 



This, then, is our fundamental hypothesis : that, as our annu- 

 late ancestors, rapacious hirudineans, grew in size, and developed 

 larger mouths and throats, a change of diet took place, in that small 

 animals and lumps of solid food were swallowed ; further, that the 

 new burden thus thrown on to the system led to a division of labour 

 between the dorsal and ventral halves, the former tending to mono- 

 polise the neuro-muscular functions, the latter the vegetative. When, 

 however, I refer to the transverse section through the trunk of a 

 vertebrate, and point to the fact that such a division of labour actu- 

 ally existed, I must not be thought to assert that this w a s brought 

 about in the manner described, only that it is conceivable that it 

 might have been so brought about. I assume that it has been so 

 brought about merely for the sake of showing that if this is granted 

 it would lead to further structural changes capable of transforming 

 our hirudinean into a vertebrate. 



In all that follows, then, we have to keep before our minds our 

 soft-bodied vermiform ancestor with a longer or shorter anterior 

 section of his alimentary canal distended by lumps of solid food ; 

 the weight of this food pressing clowirvvards, the dorsal muscles would 

 be slightly easier than the ventral, which would be seriously incapaci- 

 tated ; hence a possible cause for the gradual differentiation of the 

 two regions, the dorsal, as already stated, tending to take over the 

 animal, the ventral the vegetative functions. 



Now it seems to me that the more highly differentiated the body 

 became in the direction suggested, the more necessary would it be 

 to protect the one division from the other. The primary division of 

 labour was supposed to be due to the constant weighting and periodi- 

 cal distension of the alimentary canal with solid food. The more 

 capacious the alimentary canal became — assuming its increased 

 development as it gradually acquired a region of its own — the 

 greater the possibility of distension. Thus the danger to the dorsal 

 region from being incapacitated by pressure from the ventral region 

 is not removed ; it is rather increased, unless the two regions are 

 mutually protected from one another. 



In the Arachnida, in which the distension is sometimes positively 



