EVAPORATION. 183 



rock, and close the opening of their shells with a lid (operculum 

 or epiphragm) often of many layers, and membranous or cal- 

 careous which evidently contributes to prevent the utter 

 desiccation of the creature. Other species again cling firmly to 

 stones or plants, where they remain for weeks, and seem to be 

 protected against the drought. Their powers of resistance, 

 however, are not perfect ; every collector knows that a certain 

 number perish annually from desiccation, and these are by no 

 means old and enfeebled individuals, which in any case were 

 approaching the end of their life, but for the most part young 

 ones, not fully grown. From this it would appear that the 

 young individuals are less able to resist desiccation than older 

 or fully grown specimens. The same phenomenon is observ- 

 able in tropical regions with an insular climate ; here the dry 

 season generally affects the land-snails in the same way as in 

 the Mediterranean province. Sometimes, however, local causes 

 counteract the influence of the dry season. Thus, for instance, 

 in a garden at Manilla in the Philippines, in the driest and 

 coldest season, I found land-snails coupling, as well as their eggs 

 and young, while in other spots the same or allied species were 

 sunk in summer sleep. This was naturally the result of the 

 increased local moisture of the air in this spot, under the thick 

 leafy shade of large trees ; nevertheless, even there, the abso- 

 lute amount of vapour in the air was considerably less than 

 during the wet season. Precisely analogous is the behaviour of 

 Land-snails in deserts, where no one would expect to see animals 

 living which part with the moisture from their bodies to a dry 

 atmosphere so readily as snails do. These Desert-snails, as it 

 appears, lead an active life only during the night or early 

 in the morning, when a heavy dew moistens the soil; the 

 moisture induced by the presence of vapour in the atmosphere 

 is, however, very soon absorbed again, and during the dry day- 

 hoiirs the snails attach themselves somewhere where they are 

 protected against desiccation. Thus the time during which 

 they can imbibe the necessary moisture is about or scarcely 

 as great for these animals as for the land snails of the Philip- 

 pines in the dry season. Still we must not overlook, in the 

 first place, that probably they may be able to obtain a greater 



