22 NATURAL SCIENCE [July 
put upon it, ze. 1f 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-inusecular 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 was 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 downwards, 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 
