SUMMER SLEEP. 121 



the optimum diminishes the vital energy of the creature more 

 and more, till at last, at the minimum, life becomes latent ; in 

 this condition the animal can live for a long time without 

 danger, since the consumption of organic nutritious matter is 

 at its minimum, or, as it would seem in eggs which survive the 

 winter, almost entirely suspended during long periods. If, on 

 the other hand, the temperature rises above the optimum, the 

 vital energies are more and more increased, often accompanied, 

 no doubt, by phenomena which seem to indicate a certain 

 deterioration in them ; but a condition of latent vitality never 

 supervenes ; on the contrary, death always follows as the result 

 of a too rapid consumption of the organic matter of the body. 

 Thus the power possessed by many animals, both warm- and 

 cold-blooded, of enduring without injury a higher temperatxu'e 

 than tha.t which kills protoplasm, remains as yet unexplained 

 even by the parallel above suggested, and the solution of the 

 problem would undoubtedly be of the highest interest. 



However, the experiments and observations that have been 

 made suffice at any rate to prove that the optimum of temperature 

 differs for individuals, and even for the different organs of one 

 individual. We have seen that the control which an Infusorial 

 has over its progressive motion is lost at a degree of heat 

 above which the actual movement itself continues to increase 

 in rapidity, and we saw too that the rotatory motion con- 

 tinues to be accelerated up to the very point at which death 

 ensues, while progressive motion ceases at a much lower degree. 

 Hence we need not be surprised to see that a rising temperature 

 sometimes produces phenomena which apparently resemble those 

 produced by increased cold. Every one. knows that Europeans 

 in tropical countries, or even in a hot summer's day in our own 

 latitudes, become very sleepy, exactly as natives of the tropics 

 do under the influence of a cold climate. In these cases we are 

 justified in assuming that the too great heat deprives us of the 

 stimulus which usually keeps us awake during the daytime. 

 An attempt has been made to refer the ' summer sleep ' ""^ of many 

 animals in hot countries, as analogous to ' winter sleep,' to a long 

 continuance of such soporific influences ; but this is certainly 

 not correct ; in all these cases it is most probable that tho 



