62 ProceeiUngs of the Roi/al Irish Acinkm//. 



are very common. Bosmina was present in the first catch taken, Febniary 

 23rd, 1910; and the number steadily increased until the'maximum was reached 

 on May 11th. From that date the numbers fell untU September 8th, aft«r 

 which no others were recorded. The ma>dmum occurs, therefore, in late spiing. 

 Wesenberg Lund records the maximum for B. longirostri-s in the Danish 

 lakes in May and the beginning of Jnne, agreeing with ours. He states also 

 that its disappearance from the pelagic region after the maximum is marked 

 by the presence in the littoral regions. Kof oid records B. Imiyirostris from the 

 Illinois Eiver. It is perennial with a maximum at the end of May or beginning 

 of June and an abmpt descent after the maximum. The numbers from 

 October to May are very small. It is veiy strikiiig that in two places so far 

 removed and so dissimilar as Kofoid's region and Lough Xeagh such similar 

 sequences should be observed. The maxima seem to occur at the same time : 

 we have the abrupt descent, and none are present at all from September 

 onwards. B. longirostris occurs generally in American waters. 



lEIilPOPiAL YAEIATION IX PI.AXKION OKGAXISMS. 



Perhaps one of the most interesting and important sides of fresh-water 

 plankton studies has been the discovery and investigation of the changes taking 

 place in the form of plankton organisms during the seasons. These changes in 

 form have been seized by systematist-s, who have made countless new species 

 and varieties on the variations presented to them. 



Diuing the last fourteen years or so, however, many workers, especially 

 those investigating the plankton throughout long periods, have supported the 

 view that the variations in shape and size could not be used as indicating new 

 species, but were simply changes in one species produced by alterations in the 

 eQ\Tromnent. This view has been proved correct in many cases by taking 

 up one organism and f oUowing its changes in the same water-area thi'oughout 

 a year. One then finds a great series of steps linking up perhaps two such 

 different forms that might weU have been termed different species. Xo study 

 of this side of the plankton can be taken up without causing one to feel the 

 utter absurdity of making new species among the Protozoa and Protophyta 

 from one single individual as has often been done. Even when a group of 

 Hke indi\-iduais is found, they may be bearing merely a local and temporal 

 change in form which does not breed true. One of the results of this work, 

 pointed out by Wesenberg Lund, is that G. 0. Sars, who founded numerous 

 species of plankton Cladoeera, has gone back to such an extent that probably 

 more than fifty of the old species and subspecies of planktonic Daphnias are 

 now referred to a single species. 



