i66 ANIMAL ECOLOGY 



co-operation on a large scale between ecologists in the field and 

 experts in museums. At the same time it is useful to know 

 how to name the more obvious species of animals, and to know 

 also where to find out general information about any particular 

 group or species. A list of the works dealing with a number 

 of groups of British animals is given in the bibliography at the 

 end of this book. The Hst is necessarily incomplete, since in 

 some cases {e.g. fresh-water planarian worms) no comprehensive 

 work has been published ; while in many others, although the 

 systematic work has been thoroughly done, the results are 

 scattered in a number of periodicals, or are in relatively in- 

 accessible foreign works, or else remain locked up inside the 

 heads of experts who have not yet had time or opportunity 

 to write the necessary monographs on their groups. Further- 

 more, in the list of works quoted, no complete treatment is 

 attempted of protozoa, parasites, or marine animals. 



6. The vital importance of good systematic work and the 

 desirability of making it as far as possible available in a simplified 

 form to working ecologists has been pointed out. We may 

 now turn for a moment to the other side of this matter. 

 It is very important that ecologists should, during the course 

 of whatever work they are doing, pay attention to the collec- 

 tion of material which can be used for systematic studies. 

 Ecologists often have unique opportunities for collecting large 

 series of animals from one place at different times, and such 

 series are often invaluable in helping to decide the limits of 

 variation of different species. It is becoming more and more 

 clearly realised that the habits and habitats of animals may 

 form systematic characters quite as important as structural 

 features, and that unless information of this type is accumulated 

 in the form of good specimens with full data about habitats, 

 etc., attached, there is no proof that one " species " does not 

 contain a number of species, differing in such ways, but not 

 in obvious structures. A striking example of this kind of 

 thing is Daphnia pulex, whose life- cycle in Europe includes 

 the formation of fertilised winter eggs which enable the species 

 to tide over the winter until the following spring. In Spits- 

 bergen there is also a Daphnia which is identical in structure 



