VII.] COMMUNAL INTENSITY. 185 



grown in quantity. These are but a few isolated ex- 

 amples out of many which might be cited to show that 

 insects take on new habits when new opportunities 

 arise or when necessity compels. The same thing is 

 true, also, of fungi, but as these do not possess the 

 power of intelligent choice and spontaneous movement, 

 the adaptations are slower than in insects. 



2. Cultivation indtices change of habit in the host- 

 plant. Cultivated plants are eminently variable ; and 

 it is apparent, and I believe probable, that varieties 

 may themselves change with age or modification of 

 environment to be congenial to organisms which they 

 once more or less completely repelled. I am inclined to 

 believe that some of the so-called blight -proof and 

 rust -proof varieties now and then advertised are really 

 measurably resistant, but after a time may become 

 amenable to attack. 



3. Cultivation presents large numbers of food- or 

 host-plants in continuous areas. "It is no doubt true 

 that insects and fungi spread more rapidly than for- 

 merly, because of the greater number and continuity of 

 orchards, just as contagious diseases spread faster in 

 cities than in the country. In the small and isolated 

 orchards of former days fungi and insects were confined 

 within closer areas. This phenomenon of rapid distri- 

 bution, due to greater extent of host -plants, may be 

 termed communal intensity.'^* The more the fuel the 

 hotter the fire. Nothing so stimulates the distribution 

 and development of organic objects as an abundant or 

 excessive food supply. The potato bug could not con- 

 tain himself when he discovered the great potato fields. 



♦Bailey, Am. Gard. xi. 682. Editorial. (1890.) 



