I'fRIODIClTY IN ORGANIC LIFE. *' 57 



until at last the insects become abundant, the range of abundance 

 varyingwith the species; some even in their maximum abundance never 

 being very common. How then can we explain these variations ? 

 Meteorological conditions are not sufficient, for the scarcity may be 

 found in periods of apparently favourable weather, or the abundance 

 may be apparent in a series of cold and otherwise unsuitable 

 'seasons. Again, the period of scarcity or the reverse continues for 

 several seasons, all of which cannot present conditions favourable 

 or unfavourable for the welfare of the species. I will illustrate this 

 by referring to a few well-known examples. In 1859, and for several 

 years afterwards, the common Lackey Moth (Bombyx neiistria) 

 was very rare in Essex. I do not think I dislodged with the beatii'iQ- 

 stick a dozen larva; in any season for certainly four or five years. 

 If this scarcity had occurred only in this northern part of the county, 

 I should not have remarked it so much ; but as I had opportunities, 

 which I fully embraced, for beating in South Essex as well, it struck 

 me as something inexplicable. After an interval of this scarcity, 

 one began again to see the larvae more frequently, and at last 

 the nests were as apparent on every hedge as in the years of 

 abundance previously to 1859. Sometimes it is said that an 

 insect is scarce in consequence of the destruction of its food- 

 plant in previous seasons by the great abundance of the larva;. 

 This may be, and is no doubt true of some species ; but to a 

 general feeder like the Lackey Moth, the explanation cannot apply, 

 as no one in this country has ever seen the whole of the common 

 trees and shrubs on which this insect feeds defoliated at one 

 time. There must be some other cause for this condition of 

 periodicity. 



Another insect, the " Small Oak-Egger " ( Eriogaster laiiestris), at 

 the time mentioned (1859) was very abundant on every white-thorn 

 hedge, and in this district a nest might be found every few rods. 

 This period of abundance having lasted for some years, a decline 

 gradually set in, until now very few nests are seen, but not having 

 time to collect, I cannot say whether the insect is absolutely rare. 

 Defoliation and the consequent starvation of the larvce cannot be 

 the cause of the scarcity, for in this district, the white-thorn hedges 

 have never in late years been defoliated, except in some limited 

 localities, and then not by this insect ; it was never in sufficient 

 numbers. 



It may seem scarcely worth while to take up your time with 



