Ixxii INTRODUCTION 



Lamarck begins his physiological section with an account 

 of the distinguishing features between the organic and the 

 inorganic. He reduces the differences to nine. An organism 

 has an individuality of its own : it is necessarily heterogene- 

 ous : it comprises solid parts and fluid parts : its different 

 parts are mutually dependent on one another : it undergoes 

 what we should now term metabolism : it grows, not by 

 external accumulation of matter, but by " intro-susception " ; 

 it feeds ; it reproduces itself ; and it dies. None of these 

 peculiarities, says Lamarck, are exhibited by inorganic 

 matter. Lamarck's next task is to differentiate between 

 animals and plants. The most important distinction, he 

 says, is that the former possess irritability, while the latter 

 do not. By irritability, he means an immediate reaction ta 

 contact of a foreign body : the point of the organism where 

 the contact occurs, contracts with neighbouring parts : but 

 upon cessation of the stimulus, the parts are immediately 

 restored to their former condition, ready to respond again 

 to any new stimulus. 



Lamarck's theory of irritability is based upon his theory 

 of " orgasm." I have already said that he regarded an 

 organism as essentially composed of solid and supple " con- 

 taining " parts, through which travel the " contained " 

 visible fluids. Now the solid and supple parts were regarded 

 by Lamarck as being permanently in a state of distention, 

 due to the presence within them of caloric and other subtle 

 invisible fluids. These fluids were " expansive " ; so that 

 the supple parts to which they had penetrated were thrown 

 into a condition of distention, or, as Lamarck calls it, 

 *' orgasm." When very intense, the orgasm gives rise to 

 erethism : when insufficiently intense, to atony. 



From his theory of orgasm, naturally flowed his theory of 

 irritability. On contact of a foreign body, the subtle invisible 

 fluids are promptly dispersed in the neighbourhood of the 

 point touched. The orgasm or distention is reheved, and 

 the parts concerned immediately collapse or contract. On 

 cessation of the stimulus, the subtle fluids promptly suffuse 



