414 



LECTURE XVIII. 



broods, and each of these larvae repeats the process ; and it may be 

 again repeated in the same geometrical ratio until a number which 

 figures only can indicate and language almost fails to express is the 

 result. The Aphides generated from virgin-parents, by this process 

 of internal gemmation, are as countless as the leaves of a tree, to 

 which they are in some respects analogous. 



But why, it may be asked, should there be this strange combination 

 of viviparous generation at one season and of oviparous generation 

 at another in the same insect ? The viviparous or larviparous gene- 

 ration effects a multiplication of the plant-lice adequate to keep 

 pace with the rapid growth and increase of the vegetable kingdom 

 in the spring and summer. No sooner is the weather mild enough 

 to effect the hatching of the ovum which may have retained its vi- 

 tality through the winter, than the larva, without having to wait for 

 the acquisition of its mature and winged form, as in other insects, 

 forthwith begins to produce a brood, as hungry and insatiable, and as 

 fertile as itself. The rate of increase may be conceived by the fol- 

 lowing calculation : — 



The Aphis lanigera produces each year ten viviparous broods, and 

 one which is oviparous, and each generation averages 100 indi- 

 viduals, 



1st generation 1 aphis produces 



hundred, 

 ten thousand, 

 one million, 

 hundred millions, 

 ten billions, 

 one trillion, 

 hundred trillions, 

 ten quatrillions. 

 one quintillion. 



If the oviparous generation be added to this you will have a thirty 

 times greater result. 



It generally happens that the metamorphosis sometimes occurring 

 after the seventh or eleventh larviparous generation takes place 

 much earlier in the case of some of the thousands of individuals so 

 propagated : just as a leaf-bud near the root may develope a leaf- 

 stem, a flower and seed-capsule, with much fewer antecedent genera- 

 tions of leaves from buds than have preceded the formation of the 

 flower at the summit of the plant ; or just as one of the lower and 

 earlier formed digestive polypes may push out a bud to be trans- 



