1917] Fossil Insects 7 



exclusion of other types. If we go into the Permian, we have 

 still another great period, in which the insects were smaller, 

 and becoming more diversified, with the Blattoids in the 

 minority. 



This does not by any means exhaust our catalogue of 

 sequences. Scudder in 1896 gave an elaborate table showing 

 that during Upper Carboniferous and Permian time there was 

 a fairly regular decrease in the size of cockroaches, so that if 

 one had a number of faunulse, the average size of the members 

 would be an index to the relative ages of the strata. Since 

 Scudder's time some of the opinions of geologists have changed, 

 and from the recent material which has come in, I do not 

 believe that this class of evidence is as valuable as it seemed to 

 be; yet it is probably not without significance. More important 

 in some respects is probably the re'lationship between the 

 Archimylacrid and Mylacrid Blattoids, two groups easily dis- 

 tinguished as a rule by characters of the venation. The 

 Archimylacrids appear to be the older, and these, along 

 with the Palasodictyoptera, abound both in Europe and Amer- 

 ica. The Mylacrids, on the other hand, are essentially Amer- 

 ican, and appear to have developed during a period when there 

 was no land connection between the Old and New Worlds. The 

 proportion of Mylacrids in a given fauna is probably highly 

 significant for stratigraphy; and the whole group emphasizes 

 the fact already suggested by other evidence, such as that 

 obtained by Petrunkevitch from a study of the Arachnids, that 

 during middle Pennsylvanian time, at least, the evolution of 

 the American fauna was wholly independent of that of Europe. 

 Thus, as we investigate these matters, we do seem to observe a 

 distinct procession of events, which cannot be without signifi- 

 cance for geology or evolution. 



The Permian, or closing period of the Palaeozoic, was marked 

 in North America by an elevation of the land surface and a 

 general reduction of temperature. This continued into the 

 Mesozoic. The new conditions appear to have been unfavorable 

 to Blattoids, and to have given opportunity for the development 

 of diverse types of smaller insects, many of which passed their 

 early life in fresh water. There was at the same time a remark- 

 able development of terrestrial cold-blooded vertebrates. The 

 new start thus made probably may be taken as representing the 

 foundation of the modern insect-fauna, though several impor- 



