588 A CENTURY OF PROGRESS IN THE NATURAL SCIENCES 



But the relationship of the groups to each other and their arrangement as a 

 series of families, or as a hierarchy of families, subfamilies, and tribes, poses 

 more difficult problems. 



The origin of fleas, too, remains obscure. There is virtually no fossil record. 

 DeGeer first recognized them as ordinally distinct from other insects. Various 

 authors would associate the fleas with, or derive them from Coleoptera, Diptera, 

 Mecoptera, Trichoptera, or Ilemiptera. Jordan (1950) presented a provocative 

 paper at the Eighth International Congress of Entomology at Amsterdam, and 

 proposed that a symposium on the origin of fleas be organized for the Ninth 

 Congress. That their association with mammals is of long standing is indicated 

 by the host-relations of some groups today : a special family of fleas on bats, for 

 example, and the so-called helmet fleas, which appear to be associated with mar- 

 supials in the Neotropical and Australasian regions. Many fleas exhibit a high 

 degree of host specificity, and it is clear that many evolutionary lines have died 

 out with groups of mammals that have become extinct. Some relict species sug- 

 gest, in tantalizing fashion, some of these losses to the flea student. The primi- 

 tive sewellel {ApJodontia rufa), besides having a parasitic beetle and two aber- 

 rant species of mites, supports four species of fleas, three of which belong to 

 monotypic genera (two of these genera might well be placed in special sub- 

 families) and the fourth species is the largest of its genus and perhaps the 

 world's largest flea ! The evolutionary picture is sketchy in the extreme and is 

 complicated by numerous examples of convergence and host-transference, all of 

 which make the study of flea phylogeny and host-relationship even more difficult. 



FOSSIL INSECTS 



F. M. Carpenter 



Harvat'd University 



Since students of insect paleontology are dependent on the discovery of 

 insect-bearing deposits, progress in this field has lagged behind that of other 

 aspects of systematic entomology. Investigations of a century ago were largely 

 concerned with insects preserved in Baltic amber and the Solenhofen (litho- 

 graphic) limestone in Bavaria, both of which had been known since the time 

 of the Roman Empire. In 1853 the amber insect fauna was in the process of 

 being described by G. C. Berendt (with the aid of Hagen and others), whose 

 two-volume treatise (1845-1856) deserves to be ranked among the great classics 

 on insects. Many Solenhofen insects had already been described by Germar 

 (1842), who then (1853) turned his attention to Tertiary insects of Germany. 

 The same year (1853), 0. Ileer published the last of his papers dealing with 

 the Tertiary insects of Oeningen and Radoboj, the whole series of publications 

 forming a volume of over six hundred pages. The Jurassic insects of England 

 were being studied by J. 0. Westwood (1854) and Carboniferous insects from 

 the Saar Basin by F. Goldenberg, who established (1854) the extinct order 



