METHODS 167 



and habitat with the European form, except that it has the 

 unusual power of forming winter eggs parthenogenetically, 

 without the necessity of fertilisation by a male. There are in 

 consequence no male Daphnia in Spitsbergen at all.^-^ Another 

 good example of the importance of field observations for dis- 

 tinguishing species is that of the British warblers, which are in 

 some cases much more easily distinguished by their songs 

 and nesting habits than by their appearance. 



Now, the systematist is not usually a trained field naturalist, 

 or, if he is, he lacks the knowledge of plant and animal associa- 

 tions which is required in order to define accurately the habitat 

 of the specimens he is collecting. The ideal procedure would 

 seem, therefore, to be that as full data as possible should be 

 entered upon labels and handed over to the systematist with 

 the specimens, and that a more detailed account of the environ- 

 ment, and in particular of the animal environment, should be 

 published by the ecologist himself, who can employ, if neces- 

 sary, some means of referring to the actual specimens collected. 



7. We have dwelt at some length on the necessity of getting 

 absolutely reliable identification wherever it is possible, because 

 mistakes in this matter are one of the most fruitful causes of 

 misunderstanding, while vagueness in description of an animal 

 may render the most brilliant observations upon its ecology 

 more or less valueless. The usual mistake made by beginners 

 is in under-estimating the number of species in a genus and so 

 becoming careless about checking all specimens obtained in 

 order to get exact identification in all cases. Thus, suppose I 

 record '' Daphnia pulex eaten by sticklebacks " ; there are 

 two quite different species of sticklebacks, the ten-spined 

 (Gasterostetis pungitius) and the three- spined (G. aculeatus)^ 

 and since I had not distinguished them, you might begin to 

 wonder whether I was aware of the existence of different 

 species of Daphnia also. This element of uncertainty makes the 

 value of the observation very small. In practice it often re- 

 quires only a very small extra amount of trouble to collect a 

 few specimens of the animal on which the observations have 

 been made, or in the case of animals like birds and fishes, to 

 look up in a book to see how many species there are and which 



