Dakin and Latahche — The Plankton of Lough Neagh. 67 



summer is another wonderful ease of adaptation. How many times have neat 

 theories been propounded to account for some features in animals which are 

 probably no advantage to them whatever, but are allowed to remain by natural 

 selection because they are bound up with some character of great importance ? 

 We have actual experiments to show that nutrition affects size. In the case 

 of Crustacea, the constitution of the external medium affects the size 

 (Woltereck is at present working at this subject). Temperature -changes 

 affect size and form. In short, we. may say that numerous temporal variations 

 may be called forth by just as diverse changes in environment, environment 

 being taken in its widest sense to include all external factors affecting 

 organisms. 



The investigations of the Scottish fresh-water lochs already referred to 

 have thrown practically no light on f orm- variation ; for, in the. only lochs 

 examined throughout the year, such changes are stated to have been exceedingly 

 small. The lochs, however, where these observations were carried out are, 

 according to James Murray, amongst the largest, while the annual range of 

 temperature-variation is low. Yet the statement is made that, in the 

 different lochs, all the different forms of Asplanehna, Daphnia,"Bosmina, etc., 

 occur. Eeferring to Ceratium hirundinella, we find, from Hewitt's paper, that 

 variations occur with temperature which do not agree with those observed in 

 continental lakes nor in Lough Neagh, and which are in opposition also to 

 the viscosity theory. All this tends to support our view that the conditions 

 and factors governing form-variation are not so simple as is generally 

 supposed. 



So far as Lough Neagh is concerned, we have not done more than examine 

 some few species to determine whether form- variation occurs; and it is probable 

 that another paper will have to be published as a separate study in this 

 connexion. On the whole, we find the normal variation similar to that 

 recorded in Danish and continental lakes. 



Fragilaria crotonensis. 



In this species we find, as did "Wesenberg Lund, great variation in size. 

 Moreover, these variations in size are present at one and the same time. 

 Taking as a matter of fact any one of our catches, there is just as great 

 variation in the size as in a year's observations of averages. We have not 

 dealt with very large numbers ; but our figures indicate that, in all probability, 

 three forms occur so far as size is concerned. It is practically impossible to 

 say, therefore, from our figures whether any seasonal change in size takes 

 place. 



R,I,/\. PROC, VOL. XXX., SECT. B, [A'] 



