ENVIRONMENTAL FACTORS 43 



factor was something quite different from any of these, e.g. 

 high lime-content of the water. There are so many environ- 

 mental factors at work, and these are so often closely inter- 

 woven in their action that it may be difficult to say exactly 

 which one is playing the vital part in determining an animal's 

 distribution. For instance, the humidity of the air depends 

 upon a number of things, such as the temperature, wind, and 

 rainfall, while these in turn may be controlled by the type of 

 vegetation or the degree of exposure. 



Suppose we take Silpha quadripunctata, a beetle which 

 occurs in England almost entirely in oak woods. Since it is 

 carnivorous, the factors which actually limit it to this type of 

 wood might be any out of a very large selection, e.g. food, 

 humidity, temperature, the right colour of background, or 

 breeding conditions involving the same or other factors. 

 Again, the northern limit of the mouse Apodetnus sylvaticiis in 

 Norway coincides roughly with latitude 62° N., in other 

 words, certain temperature conditions. But another factor 

 which is correlated also with latitude is the length of the night, 

 and since this species of mouse is nocturnal it might be very 

 well limited by this factor, since in latitude 62° the night only 

 lasts five hours in the height of summer, and there is conse- 

 quently only a very short period of darkness in which the mouse 

 can feed. Without further work upon the general ecology of 

 Apodemiis we cannot say which of these factors is the important 

 one limiting its range, or whether either of them is the real 

 reason. For this reason it is fairly useless to make elaborate 

 " laws of distribution " based entirely upon one factor like 

 temperature, as has often been done in the past. It is too 

 crude a method. 



12. In field work there is not usually time or opportunity 

 for studying the real limiting factors of species ; and so descrip- 

 tion of habitats resolves itself into an attempt to record any 

 condition with which a particular species appears to be con- 

 stantly associated, even if it is a condition which is only 

 correlated with the real limiting factor or factors, and has no 

 significance in the life of the animal. If this is done it is 

 possible for other ecologists to get at least a clear idea of the 



