CHAPTER XVI 



THE GEOLOGICAL RECORD 



It is not to be expected, either from their structure or their 

 habits, that the Insecta could ever have yielded a fossil record 

 commensurate with their great numerical preponderance in the 

 Animal Kingdom. Scarcely any are marine, and comparatively 

 few are aquatic. Consequently, the record is only a partial one. 

 In the case of fresh- water deposits, it embraced only those groups 

 that were wholly or partly aquatic, or others that lived and died 

 amongst dense vegetation, which later on went to form plant- 

 bearing beds, such as our coal-measures. Waifs and strays from 

 land-living groups were also often washed down and embedded 

 in lacustrine deposits, and even in shallow marine beds formed 

 from fine mud derived from adjacent land-areas. 



Undoubtedly the most complete record furnished by any group 

 of insects is that of the Cockroaches. No aquatic group furnishes 

 us with a record which can in any way approach that of these 

 dwellers in damp and rotting vegetation. But, poor as it must 

 be judged by comparison, the record of the Dragonflies is more 

 complete than that of any other aquatic group. The actual 

 number of fossil specimens is small ; but the ancestry thereby 

 made known to us is a very long one, and very fairly complete. 



Chitin, which forms the hard parts of all insects, is very resistant 

 to strong reagents, such as acids and alkalies. But it is nevertheless 

 slowly dissolved by water. Hence it is not found in the fossil 

 state, except in those cases where it has been protected from the 

 action of this liquid, as in the case of the insects enclosed in Baltic 

 amber. Either it becomes impregnated with, and finally replaced 

 by, another substance, or else it simply dissolves away, leaving 

 only an impression of the living organism. Most insect fossils 



