426 INSECTA. 



appendages in this group, the simplicity of their metamorphosis, 

 and the presence of abdominal appendages (fig. 192), all tell in 

 favour of this view, while the resemblance of the adult to the 

 larvae of the Pseudoneuroptera, etc., points in the same direction. 

 The Thysanura and Collembola are not however to be regarded 

 as belonging to the true stock of the ancestors of Insects, but as 

 degenerated relations of this stock ; much as Amphioxus and 

 the Ascidians are degenerate relations of the ancestral stock of 

 Vertebrates, and Peripatus of that of the Tracheata. It is 

 probable that all these forms have succeeded in retaining their 

 primitive characters from their degenerate habits, which pre- 

 vented them from entering into competition in the struggle for 

 existence with their more highly endowed relatives. While in a 

 general way it is clear that the larval forms of Insects cannot be 

 expected to throw much light on the nature of Insect ancestors, 

 it does nevertheless appear to me probable that such forms as 

 the caterpillars of the Lepidoptera are not without a meaning in 

 this respect. It is easy to conceive that even a secondary larval 

 form may have been produced by the prolongation of one of the 

 embryonic stages ; and the general similarity of a caterpillar to 

 Peripatus, and the retention by it of post-thoracic appendages, are 

 facts which appear to favour this view of the origin of the cater- 

 pillar form. 



The two most obscure points which still remain to be dealt 

 with in the metamorphosis of Insects are (i) the origin of the 

 quiescent pupa stage ; (2) the frequent dissimilarity between the 

 masticatory apparatus of the larva and adult. 



These two points may be conveniently dealt with together, 

 and some valuable remarks about them will be found in Lubbock 

 (No. 420). 



On grounds already indicated it may be considered certain 

 that the groups of Insects without a pupa stage, and with a larva 

 very similarly organised to the adult, preceded the existing 

 holometabolic groups. The starting-point in the metamorphosis 

 of the latter groups was therefore something like that of the 

 Orthoptera. Suppose it became an advantage to a species that 

 the larva and adult should feed in a somewhat different way, a 

 difference in the character of their mouth parts would soon make 

 itself manifest ; and, since an intermediate type of mouth parts 



