176 ANIMAL ECOLOGY 



when they refer to years of very great scarcity or very great 

 abundance. In intermediate years they are almost, if not 

 quite, valueless. 



17. The best method of recording the relative changes in 

 numbers of fluctuating animals appears to be as follows : the 

 numbers in any one year are referred to the abundance of the 

 previous year. Thus we might say " small tortoiseshell 

 butterflies more abundant this year than last year." If a 

 continuous series of such records be made, we can then get 

 a very clear idea of the relative abundance from year to year, 

 and if there is any regular periodicity in the numbers, the 

 maxima and minima will be quite easily distinguished. The 

 advantage of this method is that it avoids the errors which arise 

 when a fictitious average ('' usual ") is used as a standard. 

 Furthermore, most people can remember pretty clearly what 

 the numbers were in the previous year, and so there is no 

 danger of introducing a great error in this way. It is advisable 

 to keep at least two separate records, one referring to the 

 breeding season, and the other to the non-breeding season. 

 Then the numbers in the breeding season of one year can be 

 compared with those both of the breeding and non-breeding 

 seasons of the previous years. The method is, of course, 

 equally applicable to monthly or other variations in numbers ; 

 its limitation is that it can only be used on fairly conspicuous 

 animals. This method of recording changes in the numbers 

 of animals requires if possible to be backed up by actual 

 census figures in some years at least. In this way it would 

 be possible to give the curve of fluctuating numbers an 

 absolute value. 



18. In conclusion, it is desirable to say something about pub- 

 lication of the results of ecological work and the best methods 

 of presenting the facts so as to be of the greatest use to other 

 people. We have already dealt with some of the more im- 

 portant errors into which it is possible to fall — insufficient 

 description of the habitat and inadequate or inaccurate identifi- 

 cation of species. There are one or two other points which 

 are worth mentioning also. The first is that primary survey 

 work and other ecological work dealing with large numbers of 



