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peated. Two new-born aphids were tried, one of which 

 produced 90, the other 49 young, but a fever prevented 

 Bonnet from carrying the trial further. 



Trembley, who was at this time living with the 

 Bentinck family at the Hague, corresponded with 

 Bonnet about the aphis-experiments, and suggested that 

 one act of fertilisation might conceivably suffice for 

 several generations. In order to investigate the matter 

 to the very bottom, he thought it desirable to isolate 

 individuals of each successive generation until either 

 males appeared or the reproductive powers became 

 exhausted. Bonnet carried out Trembley's suggestion, 

 and reared in succession five generations of the aphis of 

 the elder, all without the participation of a male insect. 

 At last the bark of the elder grew too hard to be 

 penetrated by the beaks of the young aphids, and 

 further propagation became impossible. 



Bonnet also observed two species of the genus 

 Lachnus, aphids which are common on the oak. They 

 are remarkable for their large size, 6 mm. [^ in.) ; one of 

 them had a beak longer than the whole body. Both 

 the winged and the wingless individuals reproduced 

 viviparously so long as the season was mild, no act 

 of fertilisation being observed. At the approach of 

 winter, however, small, active, winged males appeared ; 

 the females were fertilised and laid eggs. These eggs 

 were kept, but they produced no young, possibly because 

 they were not kept till spring. Trembley soon wrote 

 from the Hague that Lyonet, " qui voit tout," had found 

 fertile eggs of Lachnus on the oak in April. Bonnet 

 was now able to show that the aphids reproduce 

 viviparously so long as food is plentiful, but that when 

 severe conditions prevail, viviparous reproduction ceases, 

 and eggs are laid, which outlast the winter ; all such 



