Dakin and Latarche — The Plankton of Lough Neagh. 35 



abstracted by a haul of the net will not be a fair sample of the constitution 

 or quantity of the plankton at the time. This was the grave objection 

 brought against the quantitative work by Haeckel in his " Plankton Studien." 

 Haeckel asserted that the planktonic organisms occurred in swarms. Hensen, 

 on the other hand, upheld the view that where hydrographical conditions 

 were imiform the plankton would be found to be distributed ixniformly. By 

 this he recognized vertical distribution, and, what most of his critics forget, 

 the possibility of a non-uniform distribution where the hydrographic 

 conditions are variable. 



It would be outside the limits of this paper to discuss the opinions brought 

 forward on this subject. Amongst the foremost workers must be included 

 Herdman; and one of the authors of this paper has spent some very 

 considerable time working with him on the marine plankton of the Irish 

 Sea. 



Herdman has demonstrated the non-uniform distribution in that area, but 

 has shown that by an intensive study with stations a few miles apart we may 

 still follow out changes in the plankton quite easily. "We must emphasize 

 the fact, however, that the Irish Sea as a whole is a stretch of moving water 

 under all kinds of variable factors. It is impossible to take the catches at the 

 same time each day without taking them under very different tidal conditions. 

 The water our catches were taken from yesterday has moved away to some 

 new place to-day. Our conditions of light at the time of taking the catches 

 may have had no effect on the plankton, which has perhaps been affected by 

 the meteorological conditions of some other place when the water was in that 

 region. Furthermore, the plankton includes numerous pelagic larvae, of 

 bottom animals which have, as everyone would expect, a very non-uniform 

 distribution. In fresh-water lakes all these sources of error are absent. "We 

 have the very best conditions for quantitative work and the elucidation of 

 plankton problems. We deal with the same water, and can follow out all the 

 meteorological conditions to which it has been subjected. 



Now, it may be said that at the present time most practical workers agree 

 with the views supported and discussed by Apstein, Lohmann, Zacharias, 

 Burckhardt, and Diffenbach, that the horizontal distribution is uniform. The 

 same applies to Lough Neagh outside the littoral regions. Thus in a lake 

 like Lough Neagh we may take our catches as giving a true picture of the 

 conditions prevailing. "Were great differences in uniformity present, they 

 would have affected the curves to an obvious extent. In any case the 

 variations in constitution of the plankton which have been taken as 

 seasonal are quite distinct from accidental variations due to inaccuracy in 

 catches. 



B.I. A, PROC, VOL. XXX., SECT, 1}, [i*'] 



