124 ON THE NATURAL HISTORY OF BEETLES 



the same description of trees. This will also bo the case even 

 although the previous crop may not have been affected in the least 

 degree by such ravages, thus clearly showing that the cause of the 

 destruction to the second crop does not lie in any sort of infection 

 or transmission of the disease (if so it may be called) from the 

 former occupants of the soil, but rather from the growth and 

 establishment of larvae in the ground itself, engendered probably by 

 the dry condition of the soil caused by the previous cropping and 

 absorption of the moisture by the numerous roots left in the ground 

 after felling. This theory is supported by the fact, that fre- 

 quently after thinning young fir plantations insects are observed 

 to attack a district where they had not been previously known. 

 Probably the dried nature of the substratum of sod, intensified by the 

 continued absorption of moisture by the old roots left in the ground, 

 and also by the sprouting of some of the hardwood stools, may 

 afford congenial habitats for the incubation and increase of these 

 obnoxious enemies to the fir tribe. Some authorities attribute the 

 appearance of insects in such cases to the harbour afforded to 

 the little animals in the decaying stumps, and to the weakened 

 growth in the young wood of the trees left in the plantation from 

 the diminution of moisture in the soil ; but it appears more pro- 

 bable that the real cause lies in the drier state of the soil itself 

 (independently of the thinning process), affording a suitable and 

 congenial site for the base of the operations of the insects, rather 

 than in the harbour afforded to them by the decaying stumps 

 of trees thinned out. Thinning tends to produce stronger instead of 

 weaker shoots of young wood upon the survivors, and the roots of 

 trees felled will remain for a year or two in a sound condition, 

 whereas the attack of the insects upon the plantation generally com- 

 mences almost simultaneously with the process of thinning. 



In further support of this proposition, it may be stated that 

 in any wood where the attacks of beetles or other insects are 

 observed after thinning, it will be found that, if there happens to 

 bo a "web bit" (i.e., a part less well drained than the rest), the 

 trees there are happily exempt from the inroads of the invaders 

 during the earliest stages of their attack. A careful observer of 

 forest economy has already observed this fact in his own expe- 

 rience.* Another fact worthy of notice here, and to which par- 

 ticular reference will be made hereafter, is that these marauders of 

 coniferous plantations seem greatly enamoured of the cut and drying 

 * W. Tivendale, Scott. ArLor. Soc. Trans., vol. vii. p. 80. 



