66 Proceedings of the Royal Irish Academji. 



Again, in the sea it is characteristic to find organisms of greater size in 

 the arctic seas than in the tropics, ivhcther pelagic or not (Molluscs, Crustacea, 

 Ascidia, etc.). That is to say, the difference in size and shape is a direct 

 result of some external force, which may or may not happen to be of slight 

 advantage from the point of view of flotation. 



Now let us look more closely at the supposed advantage. The workers 

 who have taken up these theories seem to have considered the organisms as 

 dead shells. As far as we know, in floating organisms the protoplasmic part 

 which is of the greatest importance in flotation or buoyancy is directly affected 

 by the constitution of the external medium. The specific gravity of Plaice 

 eggs depends directly upon the specific gravity of the external medium. At 

 the same time it has been shown by Ostwald that changes in the specific 

 gravity of fresh water caused by temperature-variations are but slight. 

 Surely, then, planktonic organisms which are actually lighter than water will 

 remain lighter than water whatever be the changes due to the small variations 

 in our lakes. Moreover, no experiments have ever shown that organisms 

 whose specific gravity is less than that of water in spring would sink in the 

 water of summer. Thus it seems to us that all organisms which float without 

 active movement must be ruled out altogether from the resistance and 

 viscosity theory. Let us look now at the organisms which sink in water and 

 are yet pelagic. They are found at the surface because of active movements. 

 Now suppose that one of these organisms sinks 2 centimetres in x seconds 

 in cold water ; the animal by active motion must lift itself 2 centimetres 

 against the resistance of the water if it is to remain floating. Suppose that 

 this organism gave rise to others which retain the same size and shape in 

 summer. On the Wesenberg Lund- Ostwald theory, which is quite correct so 

 far as this point is concerned, the animal would sink more than 2 centimetres 

 in X seconds because the resistance of the water was less. At the same time 

 this might not be disadvantageous so far as floating was concerned, because 

 the animal could keep itself up with less energy for the same reason — the 

 reduced viscosity of the water. 



Against all this theory we have the actual experiments of Woltereck, 

 Sachse, Diffenbach, and others, which show that given a constant temperature, 

 different-sized organisms can be produced by differences in nutrition. We 

 know that temperature of the water can afi'ect shape. Wesenberg Lund's 

 discovery and that of most observers, including ourselves, show that many 

 fresh-water organisms are reduced in size in the summer months, and further- 

 more that the changes are most marked where temperature- variations are 

 greatest. This, however, simply means that growth is greater in the colder 

 water, and we must not jump to the conclusion that the smaller size in 



