79 



velopment, and the abundance was thus due to a combination 

 of favourable circumstances. 



We therefore arrive at the conclusion, that the existence of 

 favourable local conditions, at a time when extensive im- 

 migration takes place, may account for cases of sporadic 

 abundance such as we have under consideration, and that a 

 similar state of things may also result directly from im- 

 migration alone. On the other hand, we have no evidence 

 that snch cases of sporadic abundance are brought about by 

 purely local causes ; indeed, the inference is that such 

 conditions, favourable to the abnormal increase of the one 

 species, would also affect the many, and that the abundance 

 would be general rather than restricted. Such cases are 

 indeed of by no means uncommon occurrence in this 

 country ; but as they do not come within the scope of our 

 present investigation, their consideration must be left for a 

 future time. 



List of Animals and Plants observed in the Leigh, Essex, 

 district, between Southend Pier and Hadleigh Castle, 

 July 25th, 26th, 27th, 1890. 



By Mr. T. D. A. Cockerell. Read August i^lh, 1890. 



AVES. 

 Perdix cinerea, Latr. 



Amphibia. 

 Bufo vulgaris, Laur. 

 Rana temporaria, \J (Mr. J. T. Carrington.) 



Reptilia. 

 Tropidonotus natrix, L. = Natrix natrix (L.) 



MOLLUSCA. 



Limncea peregra, Miill. Scrobicularia piperata, Bellon. 



Helix cantiana, Mont. Littorina liiorea, L. 



Patula rotundata, Miill. Cardium edule, L. 



Hyalinia cellaria, Miill. Mytilus edulis, L. 



Helix virgata, DaC. Tellina balthica, L. 



„ „ var. subdeleta, Ckll. Nassa reticulata, L. 



Helix caperata, Mont. Mya arenaria, L. 



„ aspersa, Miill. 



