42 ANIMAL ECOLOGY 



where they are not found at present if the latter contained a 

 sufficiently high content of calcium carbonate, a substance 

 which the crayfish needs for the construction of its exo- 

 skeleton.106 in this case, a study of the oxygen requirements 

 of the crayfish might throw practically no light on the reasons 

 for its distribution. There are many instances of herbi- 

 vorous animals which follow their food-plant into widely 

 different climates from those to which they are accustomed, 

 when the plant itself spreads into a new country. It may be 

 taken as a rule that animals are never fully utilising all their 

 possibilities, owing to the presence of a few limiting factors. 

 In fact, an animal is limited by the things at which it is least 

 efficient, and if these disabilities are removed it can immediately 

 occupy a new range of habitat. We might put this in another 

 V way and say that in order to occupy a new environment the 

 animal has only to alter one or two of its psychological or 

 physiological characteristics. This idea is of some importance 

 from the point of view of theories of evolution and adaptation. 

 II. We may now consider some examples of the way in 

 which environmental factors limit the distribution of wild 

 animals. Very frequently when a particular species is being 

 studied, it is found that the distribution of the animal is cor- 

 related with some definite feature in its environment, but it 

 must not be assumed from this evidence that that factor is 

 the actual limiting one which is at work. This was clearly 

 seen in the case of Eurytemora, in which salinity at first sight 

 appeared to be the limiting condition in its distribution, 

 whereas the factor was really a different one which happened to 

 be correlated with the salinity. This principle is of very wide 

 application in ecological work. The habitat of any animal 

 can only be accurately described in terms of actual limiting 

 factors, but in practice we have to describe it in a very rough 

 way by means of other factors which are correlated with the 

 really important ones. When we say that an amphibian lives 

 in caves, that is only a way of indicating that it lives under a 

 set of environmental conditions of light, heat, and food, which 

 can be roughly described as cave conditions. In such a case 

 careful investigation might show that the real determining 



