232 H. H. ROSS 



cally the present. Undoubtedly dispersals between the same areas 

 occurred many times, each time involving different ecological 

 conditions in the bridging area and resulting in the spread of differ- 

 ent lines of insects having corresponding ecological adaptations. 



We know far too little concerning this historic process, however, 

 to be able to detail the spread of all the insect species in these 

 northern communities, or even that of the plants and other animals. 



Because fossils of northern insects are rarities, we must rely 

 chiefly on the biogeographic analysis of living species in delving 

 into the past history. Two sets of facts constitute the backbone of 

 such an analysis, first, the morphological characters of the species 



Fig. 1. A Baltic Amber caddisfly: right, Lype sericea and left, one of 

 its living counterparts, the European Lype phaeopa. (From Ulmer, 1912, 

 and Mosely, 1939.) 



on which phylogeny is based, and then the known geographic 

 distribution of the species from which the dispersal pattern may be 

 deduced. When present, fossil evidence is a welcome and valuable 

 adjunct to these data. 



One fact of inestimable importance in attempts at phylogenetic 

 reconstruction is deduced from the fossil record, namely that in 

 some insect lines evolutionary change may be very slow. The 

 caddisfly genus Lype is represented in both the Baltic Amber 

 (probably deposited no later than upper Eocene) and present day 

 biotas. One of the Baltic Amber species is almost identical in 

 diagnostic characters of the male genitalia with a species existing 

 today (Fig. 1), and only slight differences can be detected between 

 these populations some 50 million years apart in time. In this slow 

 rate of evolutionary change found in some of their phyletic lines, 

 the insects resemble the freshwater Pelecypoda and certain other 



