38 ANIMAL ECOLOGY 



which are dried out and cleaned every few weeks to remove 

 the conferva which grows upon them. Diaptomiis does not 

 live in the filter-beds because its eggs cannot withstand being 

 dried up, but there are swarms of Eurytemora lacinulata taking 

 its place. In this case the possibiHty of dispersal causing the 

 diflferent distribution is removed, and we have practically a 

 controlled experiment. 



6. The upshot of all this is that E. lacinulata appears to be 

 limited in its inland distribution to ponds which do not contain 

 Diaptomus. Returning to the original stream near Liverpool, 

 it now seems very likely that there are biotic factors at work 

 limiting the distribution of the Eurytemora, since we have 

 knocked out salinity as a possibiHty, and shown that competi- 

 tion is probably an important factor elsewhere. Now, as 

 Diaptomus does not occur in the stream, some other animal 

 must be acting as limiting factor or competitor. So far the 

 inquiry has not been pushed further, but the example has been 

 given in order to show that it pays to make the investigation 

 as wide as possible in order to find what is probably the best 

 line to follow up. In this case one would concentrate upon the 

 animal interrelations of Eurytemora, knowing that there was 

 a good chance of solving the problem along that hne. 



7. There would still remain the question of the limiting 

 factor or factors of Eurytemora at the other end of its range. 

 This has not been studied on E, lacinulata^ but some work 

 has been done upon E. raboti, and this will serve to illustrate 

 methods, although of course no conclusions based on one 

 species must ever be applied to another. (There have been 

 not infrequently cases in which some one has repeated a 

 man's work in order to confirm his experiments, using for his 

 material not the original species but a closely allied one, and 

 has been able to contradict the conclusions of the first man — 

 not unnaturally, when we realise how enormously closely 

 allied species may differ in their physiological reactions.) 



Eurytemora raboti inhabits weak brackish water in tidal 

 lagoons on the coasts of Spitsbergen, and sometimes when 

 these become cut oflF from the sea by rising of the land, the 

 copepods survive successfully in the relict lagoons, which then 



