112 [October, 



cws, which M. Lichteustein thiuks oviposits solely (?) on the evergreen 

 oak (Quercus ilex). Further, he thinks that the insect leaves the ilex 

 for Q. rohur and Q. puhescens, to return to the ilex once more. As the 

 latter tree is not indigenous to Britain, to Sweden, and to E. Asia ?, it 

 is obvious that in these countries some other nidus is found by the 

 insect. 



I am not quite clear as to the gist of M. Lichtenstein's question 

 upon the ho^-Aphis. He seems to ask why, if I have given three 

 pages on the extermination of this Aphis, I have not tried to follow 

 the insect from Ilumulus to Primus. He assumes, hypothetically, that 

 Phorodon malaheb is simply a different stage of P. humuli. 



I conceive that the chief part of a monograph is to gather in one 

 the scattered observations of many, and that if there be incorporated 

 original woi'k, it will appear only as an adjunct, and not as a necessity. 



Hitherto, I have regarded Phorodon malaheh as a variety of P. 

 humuli, but an intelligent correspondent, who is a large cultivator of 

 hops for the market, regards these two insects as distinct, and he states 

 that Pi'unus malaheh and P. spinosa (the sloe or black thorn) are often 

 quite absent from the grounds where the bop is grown. 



In the few experiments that I have made on Aphis rwnicis and 

 Aphis papaveris, I have failed to cause the Aphides previously nourished 

 on one genus of plant to change their food to that of another ; and I 

 am permitted, as relevant to this matter, to state the same negative 

 result from observations made during the present year, by Miss E, 

 Ormcrod, who watched some hop-plants which had twined round the 

 young suckers of the garden-plum. The hop-plants were much in- 

 fested by ^j9/^?(/fs ; but the leaves of the plum reuiained free from 

 their attack to the last. 



Though these results are negative, they are good evidence, so far 

 as they go, and they tend rather against than for the theory of 

 periodic migration, or else they would show that these insects are dis- 

 tinct in species. 



The processes of science are essentially tentative, that is, they are 

 experimental. Hypothesis accordingly pushes into theory, and theory 

 progresses into ascertained fact. No one more than my friendly critic 

 will deny the value of the scientific sieve for the separation of the real 

 fruits of observation. My foregoing remarks arc offered in no captious 

 spirit, and I know he will take them as materials for discussing a 

 problem in entomology, to which he has lent so much interest. 



Weycombe, Ilaslemcrc : 



September 6t/i, 1883. 



