DEAN : PERIODIC VARIATION IN LIMN.>EA PEUEGER. 5I 



a more difficult one. For it is here that we have to deal with those 

 localities which are liable to the greatest extremes in temperature and 

 vegetation growth and subject to agricultural interference. 



The life-history, however, of certain colonies of large acuminate 

 and ovate forms seems to be the same. Taking that history forward 

 from the time of the appearance of the large form, we find egg-masses 

 beginning to be deposited at the end of March or beginning ot 

 April, and the young grow during the first year a shell of from 3 — 3 J 

 whorls. A slight further increase* takes place the following spring, 

 but now note the discrepancy in size coupled with the following fact: 

 We find these small individuals — some of them only one-third the size 

 of their parents — depositing egg-masses, and many of them forming 

 a lip to the shell. 



The following statistics from two quite parallel cases are from 

 averages : — 



Size of shell in 1906 1907 1908 1909 



ovata - 20 mm. - 10 mm. - >L - 23 mm. 



acuminata 20 mm. - 7 mm. - >l - 18 mm. 



A division in the colony now seems to take place, and a period of 

 high mortality follows, what we may term the intermediate period of 

 reproduction. At such a time a collector will be very puzzled to 

 know whether or not he is examining mature pereger, and he will 

 incline to the belief that the shells are full-grown because a certain 

 number show a well-defined lip. 



To distinguish between the two forms, we may call the larger the 

 "Alternates" and the small intermediate form the "Revertives." The 

 idea being that these latter represent a force which reverts to the 

 normal type, and, further, that it is because of the presence of this 

 force that we have arrested variation in freshwater moUusca. 



Now follows the most vital period in the life-history of the colony. 

 The struggle for existence is becoming acute, and apparently the 

 battle is all in favour of the young of the new generation. Their 

 development is, at any rate, on a different scale to that of the pre- 

 ceding and still existent generation. For it must be remarked that 

 each appearance of the mature Alternates is followed immediately 

 by a multitudinous progeny, and if we consider the restricted nature 

 of many localities, we may find that the growth and consequent 

 development of individuals is inversely proportionate to the number 

 present ; and, conversely, that the Alternates attain their finest 

 development at a tmie when that number is below the normal. 



The next season shows an apparently inexplicable state of things, 

 if contrasted with that of two seasons ago, or if judged apart from 

 the preceding facts. 



