THOMAS ON OETHOPTERA OF DAKOTA AND MONTANA. 501 



To the first of these questions we may answer briefly, the spread of 

 the population has brought these irruptions more into notice than for- 

 merly, and from this cause they appear more frequent. But this does 

 not fully account for all the facts, and therefore we must suppose that 

 a succession of similar climatic conditions has caused more frequent 

 migrations; and that with the changes in this respect there will be a 

 corresponding change in the effect. In answer to the other questions, 

 we may state that unless there should be a remarkable permanent 

 change in the climate, we need have no fears of such results. 



