THE PLASTICITY OF LIFE 207 



In the fourth place we may recognise a group of influ- 

 ences, much larger than might be supposed at first sight : 

 those which one living creature may exert upon another 

 by contact, pressure, irritation, poisoning, parasitism, and 

 so on. We may refer, for instance, to the hundreds of 

 different kinds of galls which are produced on plants as 

 reactions to a variety of stimuli, usually the salivary 

 secretions of the larvse of small Hymenoptera and Diptera 

 that have hatched within the leaf or twig. Or we may 

 mention the strange effects that follow the establishment 

 of the Sacculina parasite in a crab, and the deformations 

 that are due to other parasites. The pearls found in 

 various bivalves seem to be the beautiful sepulchres of 

 larval tapeworms or flukes, which the skin of the mollusc 

 smothers in layer after layer of translucent lime. 



We have given only a few instances of modifiability 

 one of the fundamental facts of the Biology of the Seasons 

 but perhaps enough has been said to serve as a basis for 

 further study. Certain general results stand out. 



It is noteworthy that young creatures, such as cater- 

 pillars and tadpoles, are far more modifiable than adults. 

 The reasons for this plasticity of youth are not far to seek : 

 the young and growing organism is less " set " ; it is still 

 arranging itself internally in many of its parts ; it has not 

 quite settled down ; it is more susceptible and less em- 

 barrassed. There seems almost no limit to the tricks 

 that may be played with tadpoles. Large portions may 

 be removed without apparent detriment, part of one may 

 be grafted on to another, their growth may be quickened 

 or slowed, and they may be kept for years from becoming 

 frogs at all. 



All of this has its suggest iveness in reference to human 

 kind. We cannot hope to make a silk purse out of a sow's 

 ear, or tares into wheat, or bricks without straw, because 



