ENVIRONMENTAL FACTORS 37 



in inland fresh-water ponds which contain no unusual amount 

 of salt in the water. For instance, it lives permanently in a 

 small pond in the Oxford Botanic Gardens. Therefore 

 fresh water by itself is no bar to the survival of this animal 

 (if we assume, as we are doing here, that there are not several 

 different physiological races of this species, which have 

 different requirements in the matter of salinity). What is 

 the cause of its absence from the fresh-water parts of the Liver- 

 pool stream ? The chief way in which the fresh and brackish 

 parts of the stream differ, apart from the salinity, is in flora 

 and fauna, the upper part having different associations of 

 animals from the lower. So perhaps there is some biotic 

 limiting factor. This idea is supported by outside evidence. 

 If we examine the cases mentioned above of its occurrence 

 inland, we notice several remarkable things about them. In 

 the first place, the ponds in which Eury femora occurs are few, 

 and these few are all artificial and fairly new, mostly under 

 twenty years of age. In the second place, it is absent from all 

 older ponds, many of which are very similar to the newer ones 

 in fauna, flora, and general conditions. But these older ponds 

 do differ in one important respect : they all contain another 

 copepod, Diaptomus gracilis, which can be shown to have bad 

 powers of dispersal, and which therefore does not reach a new 

 pond for some time, usually not less than thirty or forty 

 years. The ponds containing Diaptomus have in them no 

 Eurytemoray and the ponds containing Eurytemora have no 

 Diaptomus. We can explain the latter fact by the bad dispersal 

 powers of the Diaptomus, but how are we to explain the former } 

 It appears that there must be some form of competition (see 

 p. 27) between the two species, in which Diaptomus is success- 

 ful. At this point we turn for further evidence to an experiment 

 carried out with great care (although unintentionally) by the 

 Oxford Corporation. It is always exhilarating to find an 

 experiment already done for you on a large scale, and in nature, 

 instead of having to do it yourself in a laboratory. There is a 

 large pond in the Oxford waterworks in which Diaptomus 

 gracilis occurs abundantly, but from which Eurytemora is 

 absent. A few yards away there is a series of filter-beds 



