98 VARIATION 



SECTION III MODIFICATION OF NORMAL FUNCTIONS 

 BY EXTERNAL OR OTHER INFLUENCES 



To what extent are normal functions dependent upon favorable 

 environment and how do they respond to changed conditions ? 

 A few notable facts will throw some light upon this all-important 

 question. 



Galls. An insect stings a plant. Under the influence of the 

 poison a gall is formed. If this gall be shown to a biologist he 

 will at once state with certainty the plant which produced the 

 gall and the insect that made the injury, so definite in form and. 

 appearance is the resulting growth and so distinct is it from 

 any normal growth of the uninjured plant. 



In other words, specialized plant tissues subjected to a certain 

 injury produce a new kind of growth almost as specific as when 

 operating under the laws of heredity. Here the functional activi- 

 ties of the plant at the affected point 'have been not destroyed but 

 permanently altered in such a manner as to give rise to new struc- 

 tures of definite form and often with specific chemical properties, 

 as in the case of nutgalls * containing 30 to 80 per cent of tannin. 

 In cases of this kind a new function has been developed suddenly 

 out of old materials, and it at once gives rise to new and distinct 

 products, both substantive and morphological. 



Abnormal overgrowth of disordered animal tissues. We have 

 just noted that vegetable tissues subjected to specific injuries 

 often suffer such a derangement of their functions as to cause 

 the production of an abnormal but characteristic overgrowth of 

 the injured part. Quite similar is the result of the invasion 

 of the animal economy by specific germs from without, as in 

 the case of Bacillus tuberculosis. Here the growths (tubercles) 

 resulting from the apparent attempt to encyst the foreign material 

 are sufficiently characteristic to serve as a name for the disease 

 (tuberculosis). 



Tumors generally, whether malignant or benign, are perverse 

 overgrowths of normal tissues of the body, whose ordinary 



1 All formulas for good black writing ink include gallnuts (or nutgalls) as the 

 characteristic ingredient. Those commonly used are produced on the oak by the 

 sting of the gallfly (Cy nips galhr-tinctor ice). 



