DIFFERENCES UNDER DIFFERENT CIRCUMSTANCES. 249 



Perhaps, however, it may be asked. Why should the 

 insect change its habits ? Several reasons might be sug- 

 gested. The prey first selected might be exterminated, 

 or at any rate diminish in numbers, and, though eacli 

 species as a general rule confines itself to one special 

 victim, some exceptions have already been noticed. 

 For instance, Spliex flaviioennis habitually preys on a 

 species of grasshopper, but on the banks of the Ehone 

 M. Fabre found it, on the contrary, attacking a field 

 cricket, whether from the absence of the grasshopper or 

 not he was unable to determine. 



Take another case. M. Fabre denies* that the 

 different species of Sphex can ever have been derived 

 from one source. Every species now, he observes, has 

 some one victim, some one insect on which it preys, to 

 which it restricts itself, and which tlie other species do 

 not attack. But " Que chassait, je vous prie, ce proto- 

 type des Sphegiens ? Avait il regime varie ou regime 

 uniforme ? Ne pouvant decider, examinons les deux 

 cas." 



He begins by supposing that with the ancestor of the 

 Sphex, *' Le regime etait varie. J 'en felicite hautement 

 ce premier ne des Sphex. II etait dans les meilleures 

 conditions pour laisser descendance prospere." Is it 

 likely then, he says, that they would have limited 

 themselves to one prey, and thus have foolishly 

 diminished their chances in life ? "Mais non," he adds, 

 in his lively style, " mes beaux Sphex, vous n'avez pas 

 ete aussi idiots que cela. Si vous etes de nos jours can- 

 tonnes chacun dans un mets de famille, c'est que votre 

 ancetre ne vous a pas enseignd la variete." 



He then discusses the alternative whether the 

 * " Souv. Entom., tioisieme se'rie." 



