86 VICTOR E. SHELFORD. 



stratum of a marsh and the free atmosphere above, comparable 

 to that found by Transeau between the lowest stratum of the 

 mesophytic forest and the open gravel slide. An inspection of 

 Transeau's data (Fig. 6) on evaporation indicates several obvious 

 cases of similarity; e. g., we note that the rate of evaporation 

 is about the same in the fresh water marsh and the open forest. 

 The data at present available do not justify definite conclusions, 

 yet it may be suggested that there are various stations in strata 

 of the different plant communities where the conditions of the 

 physical factors are essentially identical, but where the necessary 

 materials of abode, especially those used in breeding, are some- 

 times wanting. Their absence alone is sufficient to prevent 

 animals of specialized habits and structures from taking posses- 

 sion of situacions otherwise entirely suitable. 



The reasons for the wide distribution of some animals in the 

 forest stages which we are considering are no doubt various. 

 For example Zonites arborens (Table I, p. 67) is rare in the 

 early stages and is confined to the lower and moister localities. 

 If Epeira domicilorum is a species of stable physiological makeup 

 we can offer no explanation for its peculiar distribution (Table 

 II, p. 69). A species may have its critical period in the early 

 spring when the leaves are off the trees and the condition of the 

 atmosphere similar in all (see Fig. 2) stages or may live at higher 

 levels in the denser and older stages, and thus be surrounded by 

 similar atmospheric conditions, but we are not warranted in 

 assuming either of these causes here. 



Another striking feature of the distribution of many beetles, 

 bugs and spiders, and Orthoptera is the fact that they are found 

 in open woods, edges of woods, on the vegetation of marshes 

 and over the water of small ponds in which vegetation is growing. 

 In this way many species are found to occur in what at first 

 appear to be very unlike situations. Lygus pratensis, Tripleps 

 insiduosus, and Euschistus variolarius which occur on the vegeta- 

 tion of the margins of swamps, of the black oak forest dunes and 

 on prairies and agricultural lands may serve as examples. Shull 

 ('n) has pointed out similar facts as one of the difficulties in 

 the way of ecological classification of Orthoptera and Thysan- 

 optera. Such species as the bugs mentioned above are said to 



