214 Royal Institution i — 



living young, but that these and their descendants, to the ninth 

 generation, preserved a similar faculty. 



Observations so remarkable were not likely to pass unheeded ; but 

 notwithstanding the careful sifting which they have received, Bon- 

 net's results have never been questioned. On the contrary, not only 

 have Lyonnet, Degeer, Kyber, Duvau, and others, borne ample testi- 

 mony to their accuracy, but it has been shown that, under favourable 

 conditions of temperature and food, there is practically no limit to 

 this power of asexual multiplication, or, as it has been conveniently 

 termed, " agamogenesis." 



Thus Kyber bred the viviparous Aphis Bianthi and Aphis Rosce 

 for three years in uninterrupted succession ; and the males and true 

 oviparous females of the A. Bianthi have never yet been met with. 

 The current notion that there is a fixed number of broods, " nine or 

 eleven," is based on a mistake. 



As, under moderately favourable conditions, an Aphis comes to 

 maturity in about a fortnight, and as each Aphis is known to be 

 capable of producing a hundred young, the number of the progeny 

 which may eventually result even from a single Aphis during the six 

 or seven warm months of the year is easily calculated. M. Tougard's 

 estimate, adopted (and acknowledged) by Morren, and copied from 

 him by others, gives the number of the tenth brood as one quintillion. 

 Supposing the weight of each Aphis to be no more than 10 1 00 th of a 

 grain, the mass of living matter in this brood would exceed that in 

 the most thickly populated countries in the world. 



The agamogenetic broods are either winged or wingless. The 

 winged forms at times rise into the air, and are carried away by the 

 wind in clouds ; and these migrating hordes have been supposed to 

 be males and females, swarming like the ants and bees ! During the 

 summer months it is unusual to meet other than viviparous Aphides, 

 whether winged or wingless ; but ordinarily, on the approach of cold 

 weather, or even during warm weather, if the supplies of food fall 

 short, the viviparous Aphides produce forms which are no longer 

 viviparous, but are males and oviparous females. The former are 

 sometimes winged, sometimes wingless. The latter, with a single 

 doubtful exception, are always wingless. 



The oviparous females lay their eggs, and then, like the males, die. 

 It commonly happens also that the viviparous Aphides die, and then 

 the eggs are left as the sole representatives of the species ; but, in 

 mild winters, many of the viviparous Aphides merely fall into a state 

 of stupor, and hybernate, to re-awake with the returning warmth of 

 spring. At the same time, the eggs are hatched, aud give rise to 

 viviparous Aphides, which run through the same course as before. 

 The species Aphis, therefore, is fully manifested, not in any one being 

 or animated form, but by a cycle of such, consisting of — 1st, the 

 egg ; 2nd, an indefinite succession of viviparous Aphides ; 3rd, males 

 and females eventually produced by these, and giving rise to the egg 

 again. 



If, armed with the microscope and scalpel, we examine into the 

 minute nature of these processes (without which inquiry all specu- 



