56 OhservcUions on the Terrestrial 



ABUNDANCE OR SCARCITY OF SPECIES VARIABLE. 



Having recently revisited old collecting grounds of 1856, 1 was 

 struck with the difference in the proportion of individuals of 

 certain species existing now, and at that time ; some which were 

 rarely to be met with in that year, are now common, while others 

 which were then met with in abundance are now rare ; and I believe 

 it to be a Avell established fact with collectors that certain species 

 become rare or disappear in certain places where they were at one 

 time common, and suddenly appear in places Avhich had hitherto 

 been well searched without affording a single example of the new 

 tenant. I am informed that certain species of Unionidae which 

 were at one time common on the Ohio river are not now to be 

 met with, while other species not known to have inhabited that 

 river by our earlier zoologists are now to be frequently found. 

 This may in part be owing to a careless search in which certain 

 species are probably overlooked ; but that species do appear, or 

 disappear, or become more "or less common in certain areas, there 

 is abundant proof A partial solution of this question regarding 

 the terrestrial pulmonates may be found in the fact that a severe 

 and long continued drought may so reduce the number of indi- 

 viduals that the few surviving ones may absolutely elude detection, 

 Avhile on the other hand, a favorable season of humidity and 

 warmth may so affect the species that its former numerical 

 proportion of individuals be at once regained. In connection 

 with this subject it is interesting to compare the changes which 

 have taken place in this State, in the greater or less abundance of 

 individuals of certain species, with their condition twenty years 

 ago. In the fourth volume of the Boston Journal of Natural 

 History, (1843) will be found a catalogue of the shells of Maine 

 by Dr. J. W. Mighels in which he remarks on the greater or less 



