304 HABIT AND INTELLIGKNCI!:. [chaf. 



A fact may be mentioned of the vegetable kingdom, 



which is strictly analogous with the power of various 



parts of the external surface of animals to assume the 



respiratory function. Leaves are the organs with which 



plants decompose the carbonic acid of the atmosphere ; 



Stems but there are some tribes of plants, of which the Cacti are 



functioiis ^ ^^^® ^^^^^ known, that have no true leaves ; and in them the 



of leaves, stems are green like leaves, and discharge the function of 



Inter- leaves. And it belongs to the same order of facts, that one 



change of secreting organ frequently shows itself able, though im- 



between perfectly, to discharge the function of another. This has 



orgaur'^ been observed in cases of disease in man, and in animals 



when one of the organs has been removed or otherwise 



experimentally interfered with.^ 



I have now brought forward a number of facts and 



arguments, which all tend to show that iqy to a certain 



point tissues and organs are capable of being formed by the 



Will action of purely physical forces on vitalized matter. But 



physical ^^ ^^ important to observe, that all the instances I have 



actions mentioned are taken either from the vegetable kingdom, or 



the oiicrin from the vegetative, or nutritive, systems of animals ; and 



of all jjQ ^^y appears conceivable by which the actions of ex- 

 structures ? J STi: J 



ternal forces on the organism can account in any similar 



way for the origin of the peculiarly animal tissues — for the 

 formation of muscle and nerve. If their origin is expli- 

 cable at all, it must, I think, be due, not directly to the 

 action of inorganic forces on the organism, but to the 

 actions of the organism itself in response to impressions 

 from without. But a satisfactory physical theory of the 

 origin of muscle and nerve is out of the question in the 

 present state of science, nor do I feel sanguine of ever 

 attaining to it. I only do not say it is impossible.^ 



1 Carpenter's Human Physiology, p. 374. 



2 I say this after reading the speculations on this subject in the second 

 volume of Spencer's Princiiiles of Biology. He admits that his theory of 

 the origin of nerve-fibres does not account for ganglia. Now nerve-fibres 

 and ganglia are always found together, in all classes of animals, and they 

 are developed together; so that no theory is good for anything that 



