KENNARD : HOLOCENE NON-MARINE MOLLUSCA OF ENGLAND. 253 



factors which, combine to produce these conditions, and for my 

 purpose it is essential to ascertain what these factors are and their 

 relative importance. The methods and times of reproduction of 

 the mollusca, the history of their early stages, their powers of 

 resistance to extremes of temperature and aridity, their food under 

 natural conditions and not in a state of captivity, their enemies 

 whether external or parasitic, how little do we really know about all 

 these things. One welcomes, therefore, all such contributions as 

 that by Dr. A. E. Boycott recently published in our proceedings 

 {Proc. Malac. Soc, Vol. XIV, 1921, pp. 163-72) and the numerous 

 observations compiled by J. W. Taylor in his " Monogr.aph ". 



I had hoped that some careful observer with more leisure than 

 falls to my lot would have published ere this his observations on 

 the results amongst the mollusca of the semi-arid conditions pre- 

 vailing in England in 1921. 



So far as my limited experience goes with the land mollusca, 

 it was only the slugs that suffered. There was a very welcome 

 decrease in their numbers, especially in the case of Agriolimax 

 agrestis (Linn.), arising, I think, from the destruction of the young. 

 Xerophiles were as abundant as ever, and other forms showed no 

 diminution when the autumn rains allowed them to resume activity. 

 Very different was it with the plants and insects, which suffered 

 greatly, and a well-known entomologist informed me that he was 

 afraid that many isolated colonies of the rarer insects had been 

 extirpated. 



With the freshwater mollusca, however, the dessication of the 

 ponds and ditches only slightly reduced their numbers. The 

 small volume of water in the Thames enabled the brackish tidal 

 water to flow far above the usual limit, a fact which was realized 

 too late at Kew Gardens. But for the locks and the strict limitation 

 of the flow, the tidal waters would have reached still higher, 

 and the destruction of the flora and fauna, extensive as it was, would 

 have been much greater. But for human interference the Upper 

 Thames would have been reduced to a series of pools connected 

 by small streams, with the inevitable effects on the unfortunate 

 mollusca. But this would have been only an episode, normal 

 conditions would soon have returned, and there would probably have 

 been no trace in the geological record. The deposits we are con- 

 sidering are certainly not the products of such episodes, but represent 

 stable conditions over a considerable period of time. 



Ecologists, too, may be asked to give more details in their 

 interesting papers. The relative abundance of species in an 

 artificial habitat is interesting as showing the presence or absence 

 of enemies such as ducks and fowls, but the development of the 

 individuals is usually ignored. This is of paramount importance. 

 It must be remembered that Ecology was founded by botanists, 

 and in plants there is a keen competition between the species. 

 This is not so with the mollusca, and facts that are important to the 

 botanist are trivial to the malacologist. 



