ENVIRONMENTAL FACTORS 39 



contain only fresh water. The only factor preventing this 

 species from living in other fresh-water ponds appears to be 

 bad powers of dispersal, or at any rate difficulty in establishing 

 itself from a few eggs carried by accidental dispersal. At the 

 other end of its habitat it disappears at a certain point where 

 the salinity at high tides is over about 9 per mille (chlorides). 

 To find out whether high salinity itself was the limiting factor 

 an experiment was tried, and it was found that the animals 

 died in a salinity of 13 per mille after about two hours, while 

 they were quite unharmed in a salinity of 9 per mille after 

 more than three hours. The experiment showed with some 

 certainty that salinity was in fact the cause of its limited dis- 

 tribution in estuarine regions. -^^ 



8. When one is studying limiting factors, it is really more 

 important to have a nodding acquaintance with some of the 

 things which are going on in the environment, than to know 

 very much about the physiology of the animals themselves. 

 This statement may sound odd, and is certainly rather in 

 opposition to much of the current ecological teaching, but 

 there is a perfectly good reason for making it. Most animals 

 have some more or less efficient means of finding and remain- 

 ing in the habitat which is most favourable to them. This 

 may be done by a simple tropism or by some elaborate instinct. 

 Examples of animals which employ the first type of response 

 are the white butterflies Pieris rapce and brassicce, which select 

 the leaves of certain species of cruciferous plants for the pur- 

 pose of laying their eggs, in response to the chemical stimulus 

 of mustard oils contained in the leaves of the plants. i^^ An 

 example of the complicated type is the African lion, which 

 chooses its lair v^th great attention to a number of different 

 and rather subtle factors. It is usually supposed that animals 

 choose their habitats merely by avoiding all the places which 

 are physiologically dangerous to them, in the same way that 

 a Paramecium turns away from certain kinds of chemical 

 stimuli in the water in which it is swimming. This is true in 

 one sense ; but the stimuli which lead an animal to keep away 

 from the wrong habitat are not usually capable of doing any 

 direct harm to it, and are much more in the nature of warning 



