HIBERNATION 359 
condition (?). Lizards and snakes retire to holes in trees, under stones, 
dead leaves, and many species may congregate together in large num- 
bers. They are in a quiescent and somnolent state, not true hibernation. 
Fish of the temperate zone do not fall into a state of complete tor- 
pidity, but their vital functions are diminished and they retire to shel- 
tered holes and cease to go abroad in search of food. In the tropics 
(Africa and India) large numbers of fish are known to survive long 
and severe droughts during which the streams and ponds are completely 
dried up. This, they do, by passing the dry season embedded in 
the mud. 
Most of the species of mollusce hibernate. The land snails bury 
themselves in the ground or conceal themselves under the bark of trees ; 
in fact in almost any sort of a cavity to be found. They close the 
mouth of their shells with a calcareous plate technically known as the 
“epiphragm,” and this is perforated by a minute hole to permit breath- 
ing. The substance forming this plate they secrete in their mouths. 
During the dry weather in summer, the snails bury themselves in the 
ground and cover the opening of their shells with this protective shield, 
but it is much thinner than the one used in winter. This they do to 
protect themselves from the drought, 7. e., by checking evaporation. 
Slugs bury themselves but do not enter into a complete state of hiber- 
nation. Fresh-water mollusce go into a state of hibernation in the fall, 
burying themselves in the mud until spring. It is believed that salt- 
water mollusce hibernate in a similar manner, but practically nothing 
is known concerning them. 
Many butterflies and moths hibernate in the perfect state as well 
as in the form of imagos, but not in the larval state (?). Most insects 
which pass the winter in a state of larve hibernate during the period 
when they can not obtain any food. Insects which hibernate do not 
pair until spring and bees do not hibernate at all. It is well known to 
bee-keepers that these insects need plenty of food during the winter 
months. 
In the seeds of plants and in the eggs of many of the lower animals, 
life may remain dormant for years in cold climates, until heat or mois- 
ture awakens them. Many plants die down, while their roots remain 
alive during the winter season (perennials and biennials), coming to 
foliage and blossom in the spring. In the same way trees shed their 
leaves in the autumn and the sap returns to the roots. Similar phe- 
nomena take place in tropical countries during the hot season, when- 
ever the amount of humidity in the atmosphere is sufficient to maintain 
perennial vegetation during the entire year. 
These phenomena in the vegetable world are regarded generally as 
being analogous to those of hibernation in animals and therefore the 
terms “ hibernation of plants ” is sometimes applied to them. 
