xxill.] THE CAUSES OF DEVELOPMENT. 297 



repetition of this process, a habit is produced of forming 

 cells ; and this becomes hereditary, so that cells continue 

 to be produced in the midst of the germinating seed or of 

 the growing embryo, and other situations where the chemi- 

 cal actions are alike in the inside and on the outside of the 

 cell, and where consequently cells could not continue to be 

 formed in the same way that I suppose them to have been 

 formed at first. 



This is a mere speculation, and, so far as I am aware, 

 has no evidence except what it derives from internal 

 probability. But there is more direct e^ddence as to the 

 origin of circulatory vessels. "We know that streams have Circu- 

 excavated their own channels, and there appears to be a y^essels 

 very strong probability that the circulatory channels in h"w 

 organisms have been formed by the flow of the nutritive 

 fluids. It is stated on the high authority of Von Bar, that 

 in the animal embryo blood appears before blood-vessels 

 are formed, and circulation begins before there is a heart 

 to propel it ; ^ and we know that the embryonic states of 

 the higher forms generally represent the permanent states 

 of the lower ones. We may consequently infer that the 

 earliest circulation was not due to the action of a heart, or 

 to muscular action at all. The circulation in air-breathing Cause of 

 plants is principally due to evaporation from the leaves, j^^ir-^ ^°^ 

 but this will not apply to aquatic or embryonic organisms, breathing 

 A physical cause for their circulation, however, may be ' 



assigned. It has been ascertained ^ that if two liquids 

 have access to the same capillary tube, and one of these 

 has a stronger afiinity than the other for the substance of 

 the tube, the one that has the strongest affinity will drive 

 the other before it. Arterial blood has a stronger afiinity 



1 Carpenter's Comparative Physiology, p- 717. 



* Carpenter's Human Physiologj-, p. 255. The discovery of thi.'i law, and 

 its application to the facts of circulation, are ascribed to Professor Draper. 

 Of course it is true of porous substances, as ■well as of capillary tubes. 



This law will account for the capillary circulation in the respiratory 

 organs, as well as throughout the tissues. Throughout the tissues, the 

 arterial or oxygenated blood is attracted by the carbon of the bod)'. In 

 the respirator)' organs, the venous or carbonated blood is attracted by the 

 oxygen of the lungs. 



