2l8 EVOLUTION, OLD AND NEW. 



appetency nor organism can claim precedence, but 

 power and desire must be considered as Siamese twins 

 begotten together, conceived together, born together, 

 and inseparable always from each other. At the same 

 time they are torn by mutual jealousy ; each claims, 

 with some vain show of reason, to have been the elder 

 brother ; each intrigues incessantly from the beginning 

 to the end of time to prevent the other from out- 

 stripping him ; each is in turn successful, but each is 

 doomed to death with the extinction of the other. 



" So inflamed tendons and membranes, and even 

 bones, acquire new sensations; and the parts of muti- 

 lated animals, as of wounded snails and polypi and 

 crabs, are reproduced ; and at the same time acquire 

 sensations adapted to their situation. Thus when the 

 head of a snail is reproduced after decollation with a 

 sharp razor, those curious telescopic eyes are also repro- 

 duced, and acquire their sensibility to light, as well as 

 their adapted muscles for retraction on the approach of 

 injury. 



"With every change, therefore, of organic form or 

 addition of organic parts, I suppose a new kind of irri- 

 tability or of sensibility to be produced ; such varieties 

 of irritability or of sensibility exist in our adult state 

 in the glands ; every one of which is furnished with an 

 irritability or a taste or appetency, and a consequent 

 mode of action peculiar to itself. 



" In this manner I conceive the vessels of the jaws to 

 produce those of the teeth; those of the fingers to 

 produce the nails ; those of the skin to produce the 

 hair ; in the same manner as afterwards, about the age 



