1907- Moffat. — The Problems of an Island Fauna. 143 



days one did not need a net — one could pick them up with the 

 finger and thumb while the}'- sat at rest on the yellow heads of 

 Hawkbit, Cat's-ear, and other composite plants. These were 

 no immigrants— they were all fresh, fine, perfect specimens, 

 that evidently had had no trouble in life, and were bent on 

 taking things easy. They were, of course, the offspring of 

 those I had seen in June, but they did not emulate their 

 parents in the cultivation of the strenuous life. They stayed 

 with us all the autumn, and I fully expected to see their off- 

 spring in the following year. When that year came, I think 

 I saw some four or five at the utmost; in 1878 I saw two, in 

 1879 one, and then none for a long series of years. The pro- 

 geny of the immigrants, which had been so enormous in the 

 first generation, had failed to leave any successors behind 

 them ; but the great quantity of those insects which appeared 

 in August, 1876, from eggs laid by parents that had flown 

 across the sea the previous June, shows that there was nothing 

 to prevent similar broods from appearing in later years, if the 

 eggs laid by the Irish-bred butterflies had been equally good. 



I should, however, state that among the great swarms of 

 butterflies that formed that " Edusa " outburst of 1876 only a 

 minority were females ; and that is a feature often observed 

 in these insect outbursts, and may represent one of the ways 

 by which Nature stops the increase of insects in the outlying 

 and less favourable parrs of their range. It is said that in the 

 south of England, where that splendid butterfly the Purple 

 Emperor [Apatura iris) reaches his outlying zone, collectors 

 take on the average ten males to one female ; and I think 

 from my own observation that five or six males to one female 

 is about the proportion of the sexes in the Brown Hairstreak 

 butterfly {Thecla betulce) in its woodland haunts in Wexford. 

 It is quite possible that the same proportions obtain for these 

 butterflies elsewhere ; but I cannot help thinking the great 

 inequality may mean that the insects have reached — as they 

 clearly have in the localities referred to — the limits of their 

 respective ranges. 



If these facts are rightly explained by my supposition that 

 fertility often decreases towards the outskirts of a range, we 

 can see at once as a consequence that island faunas will gene- 

 rally be poor, Take the case of a species with a large con- 



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