1889] 



THE YOUNG NATURALIST. 



97 



been utterly exterminated by the many insect and other foes which 

 constantly find it so helpless a prey. But what I wish more especially 

 to consider is that theory of migration, either voluntary or involuntary, 

 in which Mr. Corbett appears to find the secret of our Galii and Edusa 

 years. I have been unable, I must admit, to entirely agree with him. 

 Let us consider the theory in its bearing on our two most noteworthy 

 cases, the two species already mentioned. 



It is evident, then, that we shall require a migration from some 

 place, presumably the Continent, either voluntary, and in obedi- 

 ence to an instinct akin to that which directs the swallow and the 

 fieldfare, or else involuntary, and caused by the only possible agency, 

 namely, the wind. But I do not think we have any warrant for 

 assuming any such thing as insect migratory instinct, except perhaps 

 in the case of locusts, and even here I fancy the principal impelling 

 reason for their notorious flights, is either the fact that having ex- 

 hausted the entire edible resources of one district, hunger drives them 

 to move on to pastures new, or else the entirely involuntary agency 

 of a strong and steady wind which drives them like clouds before its 

 path. Instinct must have some sufficiently compelling motive to lead 

 to its exercise : we can find none in the case of lepidoptera, it is 

 obviously not the need of food, still less the advantage derivable by 

 offspring, nor do we think we have any recorded observations sufficient 

 to justify its assumption. 



Mr. Corbett refers to the great swarms of Cardui and Gamma on the 

 Continent. It was in the summer of 1879 that one of the most notice- 

 able of these appeared, and I happened to be on the Continent that 

 year, and observed them from Germany northwards into Sweden. 

 I remember seeing the theory broached somewhere at the time that a 

 vast army of these two species, bred somewhere in the south of 

 Europe, had swept northwards across the Continent until presumably 

 they died out among arctic snows in the late autumn. But to con- 

 firm this, the further north the insects travelled the more battered 

 and worn should they have been. My observation did not bear 

 this out, the specimens in Sweden were just as bright and fresh as 

 those in Germany. My belief is that the emergences, although not 

 coincident, were distinct, through all the countries in which they were 

 seen. 



But to return to Galii in England. Even if we can accept the 

 instinctive migration theory at all, we have still to apply it to the 

 requirements of the case. The only possible origin of the immigrants 

 could have been the coasts of France, Belgium, or Holland, therefore 



