Dakin and Latahche — The Plankton of Lough Neagh. 65 



is for the purpose of increasing the superficial area relatively to the volume, 

 and hence increasing the resistance due to the viscosity of the water. 



As Weseuberg Lund has since pointed out, Ostwald's theory had been 

 put forward before that time by two other workers, 0. Miiller and Krogh. 

 Krogh, however, believes that changes in the viscosity are not the main cause 

 of seasonal variations, and takes up the view that seasonal variations are 

 occasioned by varying condition of nutriment. He has been supported by 

 Langhans, Steuer, and more recently by "Woltereck and his pupils, who have 

 experimented in great detail on Cladocera and Rotifers. Woltereck's work 

 in particular has thrown great light on the biology of the Daphnids. 



It seems to us that the satisfaction with which the theory of viscosity and 

 the relation of form to changes in viscosity has been received is tending to keep 

 back experiments on the subject. The occurrence of form-changes, which go 

 hand in hand with temperature-changes, and the delightfully simple way of 

 correlating the two by considering the changes in form as adaptations called 

 forth by alterations in viscosity, may be nothing more than building up a 

 structure on a mere coincidence. We do not want to deny absolutely the 

 theory of Wesenberg Lund-Ostwald, but we wish to show how the whole 

 theory must be considered as not proven and the necessity for further 

 observation. 



In the first place, it is difficult to see how changes in viscosity could call 

 forth a different shape or size as an adaptation. Natural selection can hardly 

 avail, because we are dealing with one species, the individuals of which are 

 affected separately and temporarily. The change in form simply means that 

 any individual may be modified by the external medium, and may itself give 

 rise to forms which may or may not be like the parent according to the condi- 

 tions under which they live. Are we to conclude, then, that this change in 

 form which is caused by some change in the external medium is an adaptation 

 because it fits in with a theory, or that it is a response to some stimulus the 

 result of which happens to have fitted in with a theory of flotation, only as a 

 coincidence ? For example, most of the pelagic eggs of fish have been found 

 to have a greater diameter as the water became less dense. For instance, the 

 eggs of the Flounder from the Baltic (in salt water) had a diameter of 1-054 

 to 1'216 mm., whilst in the more saline water of the North Sea the eggs of 

 the same species possess a diameter of 0'915 and 0-970. The specific gravity 

 of these eggs is nicely adjusted to that of the water, and yet the figures above 

 are in striking contrast to what should be expected on the Wesenberg Lund- 

 Ostwald hypothesis. We should expect smaller eggs — eggs with a greater 

 superficial area to the volume — in the less viscous water. The opposite is 

 the case. 



