Nature's Workshop 65 



growth-circle for this particular year. Of course, by that 

 time there will remain no recollection of the "caterpillar 

 plague " ; and perhaps some sapient woodman of that distant 

 future, in drawing the attention of a class of students to the 

 abnormally narrow ring, may be tempted into enlarging 

 upon the bad season, or the sunless summer, through which 

 the trees must have passed in order to account for their lack 

 of growth ; but will he ever guess that the material which 

 ought, in the ordinary course, to have gone to the formation 

 of so many tons of hard wood, has been used up in Dame 

 Nature's workshop in the manufacture of such unsubstantial 

 things as moths ? 



It is in the opinion of the writer more probable that the 

 periodic increase of insects like the Diamond-back Moth, 

 which from time to time inflicts such serious loss upon the 

 country, may be more satisfactorily accounted for in some 

 such manner as that above indicated than by the introduction 

 of the theory of immigration from abroad. That invasions 

 from over-sea do actually occur in the case of many insects 

 e.g. the Painted Lady and Clouded Yellow Butterflies, 

 some of the large Hawk Moths, Locusts, Dragon Flies, etc. 

 is of course well known ; but it is, on the other hand, 

 equally certain that the phenomenal rate of increase in some 

 cases is due entirely to local causes. Great and sudden 

 increase must obviously take place somewhere before a 

 " plague " of any insect can arise, and, that being granted, 

 there does not seem to be any very obvious reason why that 

 increase (when it occurs amongst insects that are normally 

 as abundant here as anywhere else) should not originate in 

 this country as easily as in any other place. 



