37 



individuals are developed from segments separated from 

 the larval polype. Analogous phsenomena, as we have 

 seen, are presented by some of the compound Hydriform 

 polypes, by Echinoderms, by certain Entozoa, by com- 

 pound Ascidians and Salpae amongst the Mollusks, and, 

 with various modifications, in other Invertebrata up to 

 species of Insecta, where, in the Aphis, the ( Luciua sine 

 concubitu' has ever excited the wonder if not the incre- 

 dulity of those who for the first time have been made 

 acquainted with the facts*. 



The value of the ( alternate-generation 9 theory is well 

 tested by its alleged power of affording the explanation 

 of this very remarkable mode of Parthenogenesis. The 

 virgin-procreation of the Aphides is described by Professor 

 Steenstrup in his work (p. 108), agreeably with the account 

 which I have already given and which is generally received. 

 He calls the virgin larviparous Aphides, Ammen, or ' wet- 

 nurses t'; says the species is propagated by e alternate gene- 



* " The multiplication of these little creatures is infinite and almost 

 incredible. Providence has endued them with privileges promoting fecun- 

 dity, which no other insects possess : at one time of the year they are vivi- 

 parous, at another oviparous ; and what is most remarkable and with- 

 out parallel, the sexual intercourse of one original pair serves for all the 

 generations which proceed from the female for a whole succeeding year. 

 Reaumur has proved that in five generations one Aphis may be the pro- 

 genitor of 5,904,900,000 descendants ; and it is supposed that in one 

 year there may be twenty generations/' Kirby and Spence, Intr. to 

 Entomology, vol. i. p. 175. 



f* The adoption of the extreme metaphor of Amme was least to be 

 expected from the physiologist who objected to the comparatively close 

 and true application of the Linnsean figure larva to the answerable 

 stage in the metamorphic parthenogenesis of the Radiata. " It is quite 

 erroneous," says Steenstrup, "to term the Scyphistoma the larval con- 

 dition of Medusa aurita, since a Scyphistoma never becomes a Medusa, 

 but is the quasi mother of some scores of them." (p. 6.) Not more 

 erroneous, however, than to term the wingless procreating Aphis a 

 ' larva/ which may be the mother of a winged Aphis, without herself 

 becoming one. Yet under certain conditions the ' larva ' does become 

 an ' imago,' as in A. Coryli and A. Quercus, e. g. It seems better there- 

 fore to extend, in like manner, the signification of the term 'larva' to 



