276 THE VARIATION OF ANIMALS IN NATURE 



it was devised. If the selection theory was supposed to account 

 for only a part of the facts of evolution, this criticism would 

 have little weight, but there are some biologists who regard 

 evolution as entirely a process of adaptation, and the diversity 

 of the animal kingdom as due to changes which have made 

 each species better fitted to its environment. 



The kinds of differences which we are now to consider are 

 those which the systematist encounters in his routine practice 

 and experience, and the problem may, perhaps, be seen most 

 clearly by considering a particular example. The wasps of the 

 family Psammocharidae (Pompilidae) form an isolated group, 

 probably best regarded as a superfamily. Almost all the 

 species paralyse spiders to store up for their young ; a few 

 species (Parrqferreola) lay their eggs on living spiders (just like 

 Ichneumonids), and one group, the Ceropalinae, is parasitic 

 on other Psammocharids. The European members of the 

 family have recently been monographed by Haupt (1927), who 

 establishes approximately 127 species within his faunal limits. 

 The following are the most important characters used in 

 separating subfamilies, genera and species. Head : presence, 

 especially in the female, of a bristle-tuft on the maxillae ; 

 shape and sculpture of the clypeus ; proportions of the 

 antennal segments ; the distance separating the ocelli from 

 the eyes, compared with that separating the ocelli from one 

 another. Thorax : shape and proportions of the pronotum ; 

 sculpture and length of the notum of the metathorax ; details 

 of wing-venation ; number and position of bristles on the 

 femora and tibiae ; presence or absence of serrations on the 

 hind tibiae of the female ; presence or absence of a comb of 

 bristles on the fore tarsi ; nature of the bristles at the apex of 

 the fifth tarsal segment ; structure of the claws. Propodeum : 

 shape, sculpture, arrangement of apical keels. Abdomen : 

 presence or absence in female of a transverse suture on the 

 second sternite ; arrangement of bristles on the sixth tergites 

 and sternite of the female ; structure of the sting-sheath in 

 the female ; structure of the apical sternites and genitalia of 

 the male. In addition, the colour of body, legs and wings, 

 the general surface sculpture and the extent of hairy (some- 

 times scaly) clothing are also utilised. 



On the basis of such characters the European Psammo- 

 charidae are divided into five subfamilies, with 5 (48), 4 (n), 



