48 



PERIODIC VARIATION IN LIMN^A PEREGER (Muller). 



By J. DAVY DEAN. 



(Read before the Society, May 12th, 1909). 



The following notes are the outcome of original observations made 

 for the purpose of finding out the causes for periodic variation in 

 certain colonies of Linincea pereger. The facts obtained would seem 

 to show that we may have here, not only an explanation of the 

 specific phase, but a clue also to the wider subject — the arrested 

 variation of freshwater mollusca. 



In the first place, the life-history of Limncea pereger is largely 

 governed by. the nature of the habitat, and it is not unusual to find the 

 finest shells in a small ditch which, in the heat of summer, is not water 

 so much as jungle — a dense patch of high grass and yellow-flag. And 

 it is found that so much does the regular and natural occurrence of 

 such extremes as this affect variation, and govern the life-history of 

 any particular colony subjected to these conditions, that we are 

 compelled to consider the aspect as one of primary importance. For 

 those colonies, so subjected to extremes, are nevertheless perfectly 

 self contained and perfectly balanced by a natural law. 



To give, then, the primary analysis so gained by the facts provided. 



I. — Unrestricted Localities : — by which is meant a sheet ot 



water which provides the species with the necessary conditions 



for life at all seasons of the year, and at which the seasonal 



variations in temperature, food supply, etc., are not sufficient 



to do more than cause a temporary or individual deviation 



from a recognized form. It is probable that under this heading 



may be classed most of the smaller short-spired forms. 



II. — Restricted Localities : — by which is meant a small water 



area which is liable to extremes of temperature, water-supply, 



sudden access of current, drought, food-supply, and seasonal 



variations which are sufficient to cause individual and racial 



variation in the form of the shell, and to affect and completely 



alter the life-history of the species. It is probable that under 



this heading may be classed the larger forms, acuwinata, oblonga, 



ovata, patuia^.eic. 



The pereger fall into two groups corresponding with these divisions, 



thus : — 



I. — The Annual forms. 



II. — -The Biennial forms, showing three phases : 

 A. — Simple alternation. 

 B. — Reverting alternation. 

 C— Arrested gradation. 



