ENVIRONMENTAL FACTORS 41 



usually occupying the extreme range of conditions in which 

 it could survive, since at the limits it is not so efficient, and 

 because the actual habitat is usually still further limited by 

 the breeding requirements. In other words, it is usually 

 possible to use the psychological reactions of animals as an 

 indication of their physiological " abilities," and to that extent 

 it is possible to solve ecological problems without knowing a 

 great deal about the physiological reasons why certain con- 

 ditions are unsuitable. We simply assume that they are 

 unsuitable from the fact that animals avoid places where they 

 occur. (This adjustment is presumably brought about by 

 the process of natural selection acting over very long periods, 

 since animals which chose a habitat which turned out to be 

 unsuitable would inevitably die or fail to breed successfully 

 unless they could leave it in time.) By making the assumption 

 that animals are fairly well adapted to their surroundings we 

 certainly run a risk of making serious mistakes in a few cases, 

 because owing to the lag in the operation of natural selection 

 (see p. 185) animals are not by any means always perfectly 

 adapted to their surroundings. But the rule is useful in a 

 general way, and may prevent an ecologist from getting side- 

 tracked upon purely physiological work which, however 

 interesting and valuable in itself, does not throw any light on 

 the ecological problems which he set out to solve. 



10. We may conclude this general discussion of the relation 

 of animals to their environment by referring more particularly 

 to the important general idea of limiting'^ factors. Animals 

 are not completely hemmed in by their environment in any 

 simple sense, but are nearly always prevented from occupying 

 neighbouring habitats by one or two limiting factors only. 

 For instance, in the example given above, Eurytemora lacinulata 

 was quite capable of living in fresh water, but was prevented 

 from doing so by some other quite different factor. It was 

 adapted to live in low salinities, but was unable to realise this 

 power completely. This may be said to apply to all animals. 

 They are limited in any one direction by one or two factors, but 

 are otherwise quite able to survive a wider range of conditions. 

 Crayfish would be able to live successfully in many rivers 



