88 
to this case in the growth of plants. Sleep is the 
appointed means of restoring bodily and mental ac- 
tivity, which after a limited continuance yields again 
to torpor, from deficient energy, and to a desire of 
the recruiting influence of sleep. In all classes of 
animals some appear to require more, some less of 
sleep. ‘The same case occurs amongst individuals of 
the human race. Some animals and plants flourish 
under degrees of heat and of cold which depress 
irritability, induce drowsiness or languor in others. 
But high degrees of heat and cold equally destroy 
activity and induce sleep in human beings and in 
most warm-blooded animals, endangering life in 
most species of every class, yet not in all of any! 
The arctic circle has its fauna. Snails which have 
been deemed to be dead have survived even after 
immersion in boiling water. Plants and zoophytes 
have been dried, and revived on being moistened. 
Fish have been frozen and restored. Insects have 
been found in Alpine and arctic snow. Birds roost 
within the arctic circle, and sleep where sleep would 
be fatal to most of the class: and the wolf of Mel- 
ville Island sleeps securely where the wolf of Ger- 
many would sleep to wake no more. ‘This notice 
of the various conditions of the sleeping propensity 
! De Soissy, Recherches Expérimentales sur la Physique des 
Animaux Mammiferes hybernans, remarks, that the greater size 
of the superficial vessels in hybernating mammalia, in connection 
with the non-coagulability of their blood, serves to explain their 
torpidity. Meckel finds the thymus, a glandular body connected 
with the trachea, to be peculiarly remarkable in animals which 
dive, burrow, or hybernate. Carus, vol. ii. p. 261. 
