less multiformity: the differentiation of each part tending ever to initiate differentiations of other parts. Not, indeed, that the implied process of direct equilibration can be regarded as the sole process. Indirect equilibration aids; and, doubtless, there are some of the modifications which only indirect equilibration can accomplish. But we have here one unquestionable cause--a cause that is known to work in individuals, changes of the kind alleged. Where, for instance, cancerous disease of the œsophagus so narrows the passage into the stomach as to prevent easy descent of the food, the œsophagus above the obstruction becomes enlarged into a kind of pouch; and the inner surface of this pouch begins to secrete juices that produce in the food a kind of rude digestion. Again, stricture of the intestine, when it arises gradually, is followed by hypertrophy of the muscular coat of the intestine above the constricted part: the ordinary peristaltic movements being insufficient to force the food forwards, and the lodged food serving as a constant stimulus to contraction, the muscular fibres, habitually more exercised, become more bulky. The deduction from general principles being thus inductively enforced, we cannot, I think, resist the conclusion that the direct actions and reactions between the food and the alimentary canal have been largely instrumental in establishing the contrasts among its parts. And we shall hold this view with the more confidence on observing how satisfactorily, in pursuance of it, we are enabled to explain one of the most striking of these differentiations, which we will take as a type of the class.