042 [December 



individuals before they had finished making themselves, in other words, 

 if Nature wanted larvse, and pupee to procreate as well as imagos, she 

 would begin by making puppe procreate. Now, even in those genera 

 where the pupa is to all external appearance almost undistinguishable 

 from the itnago, such as the short-winged Grasshoppers, I know from 

 long observation that the pupae never copulate. Neither is there any 

 case in any other family of insects, where it has been proved that true 

 pupae copulate; for Westwood has shown satisfactorily, that certain 

 apterous dimorphous forms in Heteroptera. which have been found in 

 copulation, are essentially distinct from the true pupfe in having no 

 rudimental wings, and are to all intents and purposes mere wingless 

 imagos. {Iiitr. II. pp. 468-70.) Again, if Nature wished to construct 

 an insect that was viviparous arid not oviparous, she would begin by 

 making the imago ovo-viviparous, and finally, after a long series of new 

 species and new genera, viviparous; and she would scarcely accumulate 

 two anomalies in the same specie.s — the anomaly of generative larvae, 

 and the anomaly of viviparous reproduction. Least of all would she 

 treble the anomaly, by superadding in the given species the necessity 

 of parthenogenetic reproduction, which appears to be the necessary 

 condition of Wagner's larvae, .seeing that no Dipterous larvae have their 

 reproductive organs developed. When Nature determined to construct 

 a viviparous vertebrate animal, she did not cause a species of some ovi- 

 parous genus of Birds to become at once, per snlfum, viviparous ; but 

 she first, by a long series of gradations which have now become extinct, 

 called into existence the Oruifhorhi/Hchus, utrne mammal which never- 

 theless lays eggs like a bird; then, by another long series of extinct 

 gradations, the Marsupials, which bring forth half-developed young, or, 

 so to speak, lay eggs which are half hatched out ; and finally, by ano- 

 ther long series of extinct gradations, the perfectly viviparous Mammals. 

 We see incipient traces of the same process in the Flesh-fly (*S'arco/>/ia- 

 (j(i) and other ovo-viviparous animals, and a more complete development 

 of it in the Dipterous Fapipara, which retain the egg in their bodies 

 till it has become a mature larva and assumed the puparium. We see 

 the first traces of the steps by which the true Mammals that suckle 

 their young have been developed out of Birds, in the Pigeons, that 

 secrete a milky substance from their craws and disgorge it into the 

 mouths of their nestlings, though we nowhere find a true Bird with 



