842 THE ZOOLOGIST. 



conception. Thus, during the nights from the 15th to the 19th 

 of August, under the favourable conditions of a south-easterly 

 wind and fair weather, a considerable migration of birds took 

 place. On each of these nights, from 11 p.m. to 3 a.m., the 

 small Noctuce above referred to were seen at the lighthouse, 

 passing from east to west in undiminishing numbers, like the 

 flakes of a dense snowstorm. These small creatures also manage 

 to cross the North Sea in safety, for they often arrive on the east 

 coast of England suddenly, and in such remarkable numbers that 

 we can only believe them to be immigrants. In fact, as my friend 

 Mr. John Cordeaux informs me, an enormous accumulation of 

 these insects actually took place at a time corresponding to that 

 of the above observations." (P. 117.) 

 Further on Herr Gatke writes : — 



" Both the theory of inheritance as well as that of tradition 

 are quite untenable in regard to the periodical migratory pheno- 

 mena of other animals endowed with the power of flight, such as 

 beetles and nocturnal Lepidoptera. Among the latter the mi- 

 grations of Plusia gamma furnish in Heligoland excellent material 

 for observation. During their autumn migrations these small moths 

 travel from Slesvick-Holstein to England, across the North Sea, 

 a breadth of water of four hundred miles in extent. They pass 

 this island in enormous swarms, resembling, as seen from the 

 lighthouse, a dense snowstorm driven by a light breeze. Thus, 

 according to an entry in my journal for 1882, on the night from 

 the 15th to the 16th of August, with a very light south wind, a 

 powerful migration of birds occurred: — 'From 11 p.m. to 3 a.m. 

 millions of Plusia gamma were travelling from east to west, like a 

 dense snowstorm.' 'Again, on the nights of the 16th, 17th, and 

 18th, large numbers of P. gamma passed the island, the migration 

 commencing each evening at 11 o'clock. On the 19th the wind 

 was south-east, the weather fine and calm. In the evening the 

 sky became over-cast, and a strong migration of birds took place. 

 From 11 p.m. until 2 a.m. thousands of P. gamma were again 

 seen.' A thunderstorm, with high winds, subsequently put an end 

 to the migration. These little insects also follow an east-to-west 

 course of migration, and they adhere to it with as much steadiness 

 and precision as the different migratory hosts of birds which are 

 observed here. That they, too, accomplish their journey in safety 

 is shown by the enormous swarms of them which frequently cover 



