402 I\Ii-. G. A. K. Marshall nn 



Hickcd thuir wings aud the movemeDt passed along the 

 row. It must be remembered that the climate of Weston 

 is extremely mild, aud the great frost of 1895 had not 

 then begun. 



The advantages of a period of rest duriug excessive 

 heat and dryness may be as great as those which follow 

 from excessive cold. In the former the food-plant may 

 be parched and dry or confined to very few and widely- 

 scattered damp spots, and the perfect insect may pass 

 through its life without the chance of laying eggs in 

 places where the larva; would be able to survive. But 

 quite apart from this, the continuous excessive drought 

 may be injurious to the perfect insect itself. At the 

 driest and hottest part of the African dry season a great 

 scarcity even of common butterflies has been noticed, and 

 it is not unlikely that man}^ individuals of some species 

 pass through the most critical period of very dry and hot 

 years concealed in a state of rest. It is significant that 

 the congregating instinct has only been observed in the 

 dry phase of Precis scsamvs. 



This does not solve the problem of the gregarious instinct 

 icself. It is clear that Dr. Jackson's observation on V. io 

 and Mr. Marshall's on Precis, etc., indicate the existence of 

 an instinct which must be a real and great danger to the 

 species. The less the individuals congregated and the 

 more widely they scattered, the greater would be their 

 chance of safety. A fortunate enemy finding one of the 

 peacocks in the bramble-bush at Weston would have 

 secured the wdiole. It is therefore certain on the principles 

 of natural selection that some great advantaue is uaiwed 



1 1 . . .® O O 



by the instmct, an advantage which more than compensates 

 for the increased danger. I would suggest that this 

 advantage is the facility given for pairing and the laying 

 of eggs without any loss of time, as soon as the period of 

 rest comes to an end. The advantage would be quite as 

 great or even greater after the rest during drought than 

 after ordinary hybernation, because of the rapiility with 

 which the food-ijlant recovers with the first moisture. It 

 would be interesting to consider from this point of view 

 the food-plants of the African species in which the instinct 

 has been observe^l. 



This suggestion naturally leads to a consideration of the 

 gregarious instinct in the peculiar form of emigration 

 which has been observed in insects. The same increased 



