174 ANilVEAL ECOLOGY 



numbers present. The method of weighing material is also 

 used. These methods are fully dealt with by Whipple,^^ and 

 by Birge and Juday.92 



One of the more important recent inventions, not described 

 in these books, is the apparatus designed by Hardy ,1^3 which 

 enables marine plankton to be collected continuously on a 

 band of silk as the ship moves along at sea. This apparatus 

 has already shown good results on the Discovery whaling 

 expedition, and will provide information of great ecological 

 interest. For by means of it, a belt-transect of the plankton 

 can be made along any desired line, and variations in the 

 fauna which are clearly shown can be used for correlation 

 with the physical and chemical gradients in the sea, or with 

 changes in the distribution of larger animals such as whales 

 and fish. 



Similar methods can be employed for soil-animals, and 

 in fact for any animals which are sufficiently small and 

 numerous to be susceptible to mechanical sampling and 

 counting. The problem becomes much more difficult in the 

 case of the higher animals like birds and mammals, which are 

 more mobile, are constantly shifting their place of abode, and 

 are, relatively speaking, so scarce as to make it impracticable 

 to kill large samples and count them. However, it is com- 

 paratively easy to make accurate censuses of nests during the 

 breeding season, and a good deal of work along these lines has 

 been done in the United States. The reader may be referred 

 to a recent book by Nicholson, ^^'^ who gives an account of 

 the methods of bird census successfully employed by him 

 in England. 



15. Grinnell and Storer^o have successfully employed a 

 different method of recording the numbers of birds. They 

 say : " Instead of using a unit of area, we used a unit of time. 

 Birds were listed, as to species and individuals, per hour of 

 observation. In a general way this record involved area too. 

 Our censuses were practically all made on foot, and the distance 

 to the right or left at which the observer could see or hear 

 birds did not differ, materially, in different regions. The rate 

 of the observer's travel did, of course, vary some . . . also, in 



