298 



HABIT AND IKTELLIGENCE. 



[chap. 



and in the 



lowest 



animals. 



Tendency 

 of circu- 

 liition to 

 form 

 channels 

 for itself. 



thau venous blood for the substance of tlie tissues ; and 

 Dr. Carpenter regards tbis action as being probably at 

 least one cause of the capillary circulation. Some such 

 action as this must probably be the cause of the motion of 

 the nutritive fluid, not amounting to circulation, which has 

 been observed in the Sertularian Hydrozoa. These animals 

 have nothing of the nature of a heart, and yet the nutritive 

 fluid, which corresponds to the blood of the higher animals, 

 flows towards growing parts, and away from dying ones.^ 



Suppose now that a movement of the nutritive fluid — 

 of sap or blood — by permeation through the cellular tissue, 

 has been commenced, whether by this means, or, as in 

 plants, by evaporation from the surfaces exposed to air 

 and sunbeams ; — and if j)roof is needed that the circulation 

 of air-breathing plants is so caused, it is proved by the 

 fact that there is hardly any circulation in the water- 

 breathing kinds ; — suppose, I say, that the circulation has 

 begun from either of these two causes, or from any other ; 

 the currents will tend to excavate channels for themselves 

 through the cellular tissue, exactly as rivers tend to ex- 

 cavate their own channels ; and tlie formation of such 

 channels will become hereditary. This is no mere hypo- 

 thesis ; it has been as directly verified as the nature of the 

 case admits of. A perfect gradation may be seen in air- 

 breathing plants, from cellular tissue to perfectly-formed 

 vessels. First, ordinary cellular tissue ; next, cells elongated 

 in the direction in which the sap flows, as determined by 

 the place of greatest evaporation ; next, cells having their 

 separating walls broken or dissolved away, so as to unite 

 them into tubes ; finally, tubular vessels showing no longer 

 any vestige of cell- structure. I do not mean that this 

 whole course of development takes place in the lifetime 

 of any one plant ; but the comparison of these transitional 

 stages makes it highly probable that such has been the 

 course of development through successive generations ; and 

 there are many cases where a part of the transition may 

 be witnessed in the individual.^ 



1 Carpenter's Comparative Physiology, p. 633. 



For details see Spencer's Princij)les of Biologj-, Part V. chap, iv 



