64 Proceedings of the Royal Irish Academy. 



The variations observed in the above cases affect the size, shape, and 

 presence or absence of longer or shorter spines. It would be quite impossible 

 here to refer to the extensive literature on the subject. Eeference should be 

 made to Wesenberg Lund (1) and to numerous papers in the Eevue Inter- 

 nationale de Hydrobiologie during the last few years. There is very much 

 work to be canied out yet on this subject; but it is necessary that observers 

 make a special study of some particular group. 



The cause of these temporal variations has been ably discussed by 

 Wesenberg Lund ; and it is to this worker that we owe the fu-st attempts to 

 explain the variations by a common phenomenon. "Wesenburg Lund's view 

 was that the organisms reacted to an external stimulus, and that this stimulus 

 was a change in the specific gravity of the water caused by changes in tempera- 

 ture. Form- variation was therefore an effort to bring the specific gravity of 

 the organism into equilibrium with the altered specific gi-a\dty of the water at 

 certain times of the year. It was observed that seasonal variations were not 

 commonly found in arctic, alpine, and other lakes where great annual 

 fluctuations of temperature did- not occur. The relation of Lough Xeagh 

 to the Scottish lochs is an example of this, and will be referred to again 

 below. 



Numerous workers have noticed that the size of many plankton organisms 

 becomes smaller as the summer approaches. Wesenberg Lund's %-iew was 

 modified and accepted more readily after the work of Ostwald. Ostwald 

 showed that the changes in specific gi-avity of the water were very small, 

 owing to the small ranges in temperatui-e, but that this temperatui-e-variation 

 strongly affected the viscosity of the water, the property possessed by a liquid 

 of resistiug movement through it, or, iu short, the friction. 



The power of flotation possessed by a plankton organism depends upon its 

 specific gra^-ity. If an organism is heavier than water, bulk for bulk, it will 

 smk. The speed of sinking depends upon the area of the surface in contact 

 with the Kquid relatively to the volume of the object, and upon the shape of 

 the sinking body. A third factor, however, stands in important relationship 

 to the specific gra^dty and shape, namely, the viscosity of the water. We may 

 experiment with the same object in two fluids having the same specific 

 gra\dty — for example, alcohol and melted wax. It ^"ill be seen clearly that 

 the speed of sinking depends on the nature of the liquid — on its Mscosity. 

 Temperature affects the viscosity ; for example, the speed of sinking is twice 

 as fast at 25' as it is at 0". The addition of salt in solution in water 

 decreases the sjpeed of sinking. These physical questions throw very con- 

 siderable light on the factors governing the biology of plankton organisms ; 

 and we might infer that the decrease in size of an organism in the summer 



