THE LAW OF EVOLUTION 



end to end a tolerably uniform series of molar 

 motions of constriction. But as the canal be- 

 comes more heterogeneous, the molar move- 

 ments in its different parts simultaneously be- 

 come more unlike one another. While the 

 waves of contraction and expansion remain 

 constant and moderate throughout the small 

 intestine, they are replaced in the oesophagus 

 by more violent contractions and expansions 

 that recur at longer rhythmical intervals. In 

 the stomach the mechanical undulations are so 

 much more powerful as to triturate the con- 

 tained food, and their rhythms are differently 

 compounded ; while the movements of the 

 mouth are still further specialized in the actions 

 of biting and chewing. In the molecular motions 

 constituting secretion and absorption there is a 

 similar specialization. While absorption is con- 

 fined chiefly to the area covered by the lacteals, 

 secretion is specialized in various localities — 

 in the salivary glands, in the gastric and intes- 

 tinal follicles, in the liver, and in the pancreas 

 — and in each place it has acquired a peculiar 

 character. A like increase in heterogeneity and 

 definiteness marks the circulatory movements. 

 In a slightly evolved animal the nutritive fluid, 

 answering to blood, moves about here and there 

 at seeming random, its course being mainly 

 determined by the local pressure of the tissues. 

 But in a highly evolved animal, which possesses 

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