42 THE ECOLOGY OF ANIMALS 



gas contents, salts, hydrogen ion concentration, and 

 so on. A large number of artificial experiments have 

 been carried out in the laboratory. In these, aU 

 factors except one are controlled and the one factor 

 varied. By such experiments it has been proved 

 that every species of animal has very definite optimum 

 conditions both for maximum survival (length of 

 life), activity, sometimes reproductive rates, and 

 population increase. Above and below the optimum 

 there is a faUing off in survival, activity, or rate of 

 increase. Beyond certain points inactivity occurs 

 and beyond that again death. The second method 

 has been to combine two factors together, such as 

 temperature and humidity. By doing this we obtain 

 what is in effect a three-dimensional diagram, which 

 can be plotted as a contoured graph. Thus in the 

 tropical plague flea {Xenopsylla cheopis) the thermal 

 death-point is 22° C. with relative humidity of per 

 cent, (completely dry), 27° C. with relative humidity 

 of 30 per cent., 32° C. with relative humidity of 60 

 per cent., and 36° C. with a relative humidity of 

 90 per cent. (Mellanby, 1932). In other words the 

 flea can stand higher temperatures in damper con- 

 ditions. This shows the manner in which one 

 physical factor conditions the action of another. 

 Or take the marine flat worm {Gunda ulvae). This 

 worm lives in the estuaries of streams running on 

 to the sea- shore. It is subject alternately to fresh- 

 and salt-water influence. Weil and Pantin (1931) 

 showed that pure fresh water killed the worms in 

 forty-eight hours, but that the onset of death (sweU- 

 ing up through intake of fresh water) could be 

 prevented by adding a certain amount of calcium 

 carbonate to the freshwater — as is the condition in 

 nature. 



A great many experiments of this type have been 

 carried out, and the results so far achieved raise a 

 hope that many features in animal distribution wiU 

 be clearly explained in these terms. This kind of 



