THE YOUNG NATURALIST. 



[September 



show to be, one not only of degree but of kind. Let me for the sake 

 of clearness use the word migration only in that more restricted sense 

 which I have indicated. 



Migration then must and does have for its object the welfare of 

 the species, and its derivation is easily accounted for by the process of 

 natural selection which has accumulated and intensified any original 

 tendency of the kind, by the consequential benefit derived by those 

 individuals in whom the tendency was strongest. But what we must 

 especially bear in mind is, that this migration involves essentially the 

 idea of alternation between relatively fixed points. The movement 

 must be recurrent, from that it derives all its value, and it seems to 

 me almost impossible to conceive, how such an instinct could have 

 arisen in beings so short lived as insects, to suggest only one point. 

 Let us take the case of the subject of the argument, D. galii. Ac- 

 cording to the evidence of the immigrationists themselves, the only 

 result accruing to that species, from its supposed tendency to 

 migration, is that the descendents of such individuals as do migrate, 

 become extirminated, now surely the action of natural selection if 

 that agency lias any meaning, would be the gradual elimination of 

 such a habit from the species altogether. I think this point could be 

 greatly elaborated, but all I wish to point out is, that although it may 

 be easily possible to explain in insects a simple tendency to wander, 

 either singly or in masses, in an indefinite direction, a tendency which 

 might be induced by scarcity of food plant, attacks of enemies, or 

 numberless other contingencies, that is a totally different character- 

 istic from what I have explained above and what I referred to when I 

 said I did not think we had any warrant for assuming its possession 

 by insects. 



I think Mr. Tutt has not sufficiently discriminated between these, 

 but allowing that he merely explains the abundance of Galii by what 

 I call a tendency to dispersion, let us see how far the facts of the case 

 will bear him out, and this brings us to a consideration of the second 

 point. 



I am glad to see that Mr. Tutt at least agrees with me, in dis- 

 missing as impossible the " wind-blown theory," but I ought to re- 

 mark in passing that the meaningless phrase " involuntary migratory 

 instinct" is Mr. Tutt's own, I never used such an expression. But 

 to proceed, Mr. Tutt " believes that the batch of D. galii captured in 



