Comparative Physiology. 195 



stand a heat of 260° appears more than lymphatic action would 

 account for. The power of assimilation is a vital affinity, which 

 has not been imitated or understood yet. The power of light 

 has been said by some to be wholly chemical, but others regard 

 it as acting greatly by the stimulus it gives to the power of the 

 secreting organs in the leaf. The gastric juice of the stomach 

 is said to act on and destroy dead matter, even the stomach 

 itself after death ; but to have no power on it wliile living, be- 

 cause protected by its vital properties. The egg resists putre- 

 faction at a great degree of heat while alive ; but, if the electric 

 spark be passed through it to destroy life, it soon commences to 

 putrefy. The blood of the animal, and sap of the plant, though 

 extravasated, will live and become organised, if connected 

 with the living system. The blood has been said to be kept 

 in a fluid state, and tendency to solidify prevented, by its 

 vital properties ; it is said to solidify if forced through dead 

 tubes. Chemistry has lately been able to imitate some actions 

 considered as vital : starch can be formed from woody fibre, 

 sawdust, bark, and other substances ; sugar and gum from 

 starch, &c. These chemical transformations have been princi- 

 pally, however, in the descending series (though woody fibre 

 has been said to have been formed from stai'ch by nitric acid 

 and chlorine), and they have been produced chiefly on what 

 are called organisable or proximate principles. It has not yet 

 been thought possible to form any of these from their elements 

 of carbon, hydi'ogen, and oxygen ; at least none have succeeded 

 in doing this, though Professor Thomson, in his Vegetable Che- 

 mistry, seems to think we may yet do so. It may be, that che- 

 mical affinity is the principal agent by which living organised 

 bodies are produced, as well as inorganic combinations of dead 

 matter. But if so, that agency is controlled by a power which 

 we can only as yet feebly imitate in the production of or- 

 ganisable proximate principles. I fear the day is far distant 

 when even these will be produced from their elements ; but, 

 though we should attain so far, when shall we hope to be able 

 to imitate the power of assimilation which can from such or- 

 ganisable products, through the mere agency of cells in which 

 no distinguishing anatomical diflerence can be found, cause so 

 many and such varied secretions and assimilations to take place, 

 and combine the whole in one system, in which all the parts so 

 mutually harmonise with each other? Such opinions as the 

 above, and others, on vitality, &c., have by some been said to 

 savour of materialism, but erroneously; as the wisdom of the 

 Creator may be as well displayed in working by one agent as 

 another. 



On the elementary structure of vegetables, he divides the 

 subject, as in other similar works, into — 1. Cellular Tissue; 



o 4 



