4 Annals Entomological Society of America [Vol. X, 



of a progressive movement, there has nevertheless been much 

 of the shuffling already mentioned, producing a great mass of 

 specific forms, while many genera have become extinct. Aside 

 from these general questions, we may value the evidence 

 afforded by fossil insects for the light thrown on geology and 

 paleogeography . In the first place, although the genera of 

 insects are of long duration, the species appear to be short 

 lived. The best evidence for this opinion comes from the fact 

 that strata supposed by the geologist to. be of nearly or quite 

 the same age, often contain insect-faunulae in which the species 

 are all distinct. This may be partly due to different ecological 

 conditions and to migrations, but it certainly is due in part to 

 the comparatively rapid evolution of insect species. This is 

 especially proved by the Pleistocene beetles studied by Scudder, 

 which are closely allied to modern species, yet distinct. Pro- 

 fessor Wickham is now engaged in the study of many additional 

 Pleistocene beetles, and though his work is not finished, he 

 kindly informs me that "they seem to be pretty nearly all 

 different, subspecifically at least, from those of today." 



Owing to the complexity of insect life and the facilities these 

 animals have for getting about, faunas are constantly in a state 

 of flux, species locally dying out, and others coming in. Thus 

 there can be little doubt that complete collections made in any 

 locality at intervals of one hundred years would be appreciably 

 different; except perhaps in the tropics, where conditions are 

 likely to be more uniform. It is doubtless on account of this 

 fact that we not rarely find non-functional examples of 

 "mimicry," which are offered as obstacles to the view that 

 mimicry has any adaptive significance. It is evident that the 

 almost kaleidoscopic insect fauna must present characteristics 

 which are to be understood in relation to the past rather than 

 to the present. It results from all these considerations that 

 fossil insects, when they can be obtained in any numbers and 

 from different levels, afford a very delicate index to the details 

 of stratigraphy, probably surpassing in this respect every group 

 of organisms except mammals, which are not available for the 

 purpose until we reach the Tertiary. The obvious objection to 

 the use of insects in this manner arises from their comparative 

 scarcity; but this has been exaggerated, and every year brings 

 to light new localities. In particular, the Pennsylvanian 

 (Upper Palaeozoic) coal bearing strata of Maryland, Pennsyl- 



