DEVELOPMENT OF CIKCULATOKY CHANNELS. 405 



These parallelisms in the evolutions and structures of 

 tne circulating systems, introduce us to otliers in the kinds 

 and rates of the movements going on through them. In 

 the lowest societies, as in the lowest creatures, the distri- 

 bution of crude nutriment is by slow gurgitations and re- 

 gurgitations. In creatures that have rude vascular sys 

 terns, as in societies that are beginning to have roads and 

 some transfer of commodities along them, there is no regu- 

 lar circulation in definite courses ; but instead, periodical 

 changes of the currents — now towards this point, and now 

 towards that. Through each part of an inferior mollusk's 

 body, the blood flows for a while in one direction, then 

 stops, and flows in the opposite direction ; just as through a 

 rudely-organized society, the distribution of merchandise 

 is slowly carried on by great fairs, occurring in diflerent 

 localities, to and from which the currents periodically set. 

 Only animals of tolerably complete organizations, Hke ad- 

 vanced communities, are permeated by constant currents 

 that are definitely directed. In living bodies, the local and 

 variable currents disappear when there grow up great 

 centres of circulation, generating more powerful currents, 

 by a rhythm which ends in a quick, regular pulsation. 

 And when in social bodies, there arise great centres of 

 commercial activity, producing and exchanging large quan- 

 tities of commodities, the rajDid and continuous streams 

 drawn in and emitted by these centres, subdue all minor 

 and local circulations : the slow rhythm of fairs merges 

 into the faster one of weekly markets, and in the chief cen- 

 tres of distribution, weekly markets merge into daily mar- 

 kets ; while in place of the languid transfer from place to 

 place, taking place at first weekly, then twice or thrice a 

 week, we by-and-by get daily transfer, and finally transfer 

 many times a day — the original sluggish, irregular rhythm, 

 becomes a rapid, equable pulse. 



Mark, too, that in both cases the increased activity, like 



