CHAPTER IV 



ENVIRONMENTAL FACTORS 



The ecologist is (i) concerned with zohat animals do, and with the factors 

 which prevent them from doing various things ; (2) the study of factors 

 which Hmit species to particular habitats Hes on the borderHnes of so 

 many subjects, that (3) he requires amongst other things a slight know- 

 ledge of a great many scattered subjects. (4), (5), (6), (7) The ecology 

 of the copepod Eurytemora can be taken as a good example of the methods 

 by which such problems may be studied, and illustrates several points, 

 e.g. (8) the fact that animals usually have appropriate psychological 

 reactions by which they find a suitable habitat, so that (9) the ecologist 

 does not need to concern himself very much with the physiological 

 limits which animals can endure ; and (10) the fact that animals are 

 not completely hemmed in by their environment, but by only a few 

 limiting factors, which (11) may, however, be difficult to discover, 

 since (12) the factor which appears to be the cause may turn out to be 

 only correlated with the true cause. (13), (14) Examples of limiting 

 factors to the distribution of species are : hydrogen ion concentration 

 of water ; (15) water supply and shelter ; (16) temperature ; (17) 

 food plants ; and (18) interrelations with other animals. The last 

 subject is so huge and complicated, that it requires special treatment — 

 as the study of animal communities. 



I. It will probably have occurred to the reader, if he has got 

 as far as this, that rather Uttle is known about animal ecology. 

 This is, of course, all to the good in one way, since one of the 

 most attractive things about the subject is the fact that it is 

 possible for almost any one doing ecological work on the right 

 lines to strike upon some new and exciting fact or idea. At 

 the same time it is often rather difficult to know what are the 

 best methods to adopt in tackling the various problems which 

 arise during the course of the work. This is particularly true 

 of the branch of ecology dealt with in the present chapter. 

 Much of the work that is done under the name of ecology 

 is not ecology at all, but either pure physiology — i,€, finding 

 out how animals work internally — or pure geology or me- 

 teorology, or some other science concerned primarily with the 



33 D 



