36 ANIMAL ECOLOGY 



the main data about the animals, one would be faced by a 

 number of inexplicable and fascinating cases of limited dis- 

 tribution of species, upon which would have to be focussed 

 every single scrap of other knowledge which might bear on 

 the problems. Having crystallised the problems it is often 

 desirable to carry out a few simple experiments ; but more 

 often, careful observation in the field will show that the ex- 

 periment has already been done for one in nature, or by some 

 one else — unintentionally. 



4. Let us take an example which will give some idea of the 

 way in which the factors Hmiting the distribution of animals 

 may be studied. To continue the same line of thought as 

 before, we will consider Eurytemora lacinulata, a copepod 

 crustacean which normally inhabits weak brackish water in 

 lagoons and estuaries. In an estuarine stream near Liverpool 

 which has been studied, E. lacinulata is absent from the upper 

 fresh- water parts, as also from fresh- water ponds in the neigh- 

 bourhood, but begins to occur in the region which is in- 

 fluenced for a short time only by salt water at the height of 

 the tides. In this part of the stream the environment consists 

 of fresh water at low tides, brackish water (e.g. up to a salinity 

 of 4 (chlorides) per mille) at high tide, with a gradation in 

 between. Further downstream, under more saUne conditions, 

 the Eurytemora again disappears. Near this stream there is 

 a moat surrounding an old duck decoy, and filled with brackish 

 water whose salinity fluctuates slightly about 4 per mille, 

 and in which Eurytemora lives and flourishes. It is clea;* 

 that in this region E. lacinulata lives in weak brackish water 

 up to about 4 per mille, or in alternating weak brackish and 

 fresh water, but neither in permanent fresh water nor in more 

 saline water. We are justified in making the hypothesis that 

 E. lacinulata requires a certain optimum salinity, any wide 

 deviation from which is unfavourable to it. 



5. Now it has been found, on the other hand, that in the 

 Norfolk Broads this species lives in the fresh-water broads 

 and is almost entirely replaced in the brackish ones by an 

 allied species, E. affinis. Furthermore, there are a certain 

 number of curious records of the occurrence of E. lacinulata 



