56 THE ENTOMOLOGIST. 



With due respect to the learning and reputation of some of 

 those who gave expression to their views upon the ahove- 

 mentioned occasion, permit me to say that personally I consider 

 the climatal conditions to have played quite a subordinate part 

 in causing the decadence of the indigenous insects under 

 discussion. 



From the point of view of an orthodox evolutionist of the 

 natural selection school, the primary factors in the matter, to my 

 mind, are insular isolation coupled with the consequent conco- 

 mitant powerful operation of the law of amixia. This hypothesis 

 is supported by the fact that nearly all the species which have 

 been extirpated or are at present on the point of extermination 

 possess comparatively weak powers of flight, at the same time 

 they do not contain in their ranks any species with a very 

 pronounced predilection for migrating. At any rate, they all 

 belong to that class which are not known to immigrate 

 periodically to this country from the Continent. On the other 

 hand, many of those species which maintain their existence 

 perennially in these inhospitable isles are frequently known to 

 come across the English Channel, occasionally in large swarms, 

 such for instance as Picris hrassiece, P. rcqne, P. napi, and 

 Vanessa atalanta ; while others, such as Vanessa cardui and 

 CoUas liyale, would not probably permanently occur in this 

 country for more than a few years at the most, were it not for 

 these remarkable peregrinating powers which they possess. 



This isolation from the Continent is therefore, in my opinion, 

 the primary cause of the decadence phenomenon under discus- 

 sion, and the law of amixia comes into operation as a necessary 

 consequence. As a rule all those species which subject them- 

 selves to its despotic influence, namely, the prevention of cross- 

 ing by isolation, have as a result the degeneration of their 

 physical powers, their fecundity undergoes a deterioration, the 

 number of ova which they produce is thus probably considerably 

 below the average, and the offspring at the same time do not 

 possess sufficient of the necessary vitality and vigour to enable 

 them to contend successfully with the numerous enemies by 

 which they are surrounded. 



Another very probable cause of the gradual extinction of 

 some of our indigenous lihopalocera is to be found in the 

 increasing number of their enemies which subsist exclusively 

 upon insects for food. Contrary to that which is the case in 

 most parts of the Continent (at least according to my circum- 

 scribed experience), the number of individuals of insectivorous 

 birds in this country is exceedingly great. This has been 

 brought about l)oth by reason of their legislative protection and 

 the merciless war waged by gamekeepers against the hawks and 

 owls which prey upon them. At the same time, from the latter 

 cause also, the number of insectivorous animals {Talpa and 



