184 THE INFLUENCE OF INANIMATE SUREOUNDINGS. 



quantity of water from their food, consisting of the fleshy-leaved 

 succulent desert-plants, than other snails ; and, in the second 

 place, that they may also be capable of absorbing a larger 

 amount of water endosmotically through the skin than the 

 snails living in our damp climates. Unfortunately, so far as I 

 know, no experiments and observations exist as to the rapidity 

 and period of growth of land-snails in countries where the 

 moisture of the aii' differs widely or varies much. 'So series of 

 systematic and accurate experiments are known to me, even 

 with reference to our commonest snails, which collect in thou- 

 sands every year, but only a few accidental observations ; '^ so 

 that the well-known statement of Agassiz — that in the shell of 

 a Helix a ridge corresponds to each year's growth, like the 

 annual ring in a tree — cannot at present be tested as to its 

 general or partial accuracy. Kesearches in this direction would 

 certainly be productive of results of universal vakie, as I am 

 justified in concluding from a few general observaUons. Mean- 

 while from the few materials at hand as to the behaviour of 

 land-snails under various degrees of moisture in the atmosphere, 

 only one conclusion may be drawn which seems highly pro- 

 bable : That the various species behave very differently iu this 

 respect, so that an alteration in the moisture of the aii- in any 

 region must fundamentally alter the Snail-fauna inhabiting it. 



Other animals perish from desiccation in quite other ways. 

 For instance, in the tropics, as well as in North America, very 

 many insects die out almost completely during the dry season, 

 which by no means always corresponds with the hottest season, 

 as it does in America. On the western side of Luzon, January, 

 the driest month, is also the coldest. Certainly even at this 

 season a number of insects are always to be found, chiefly 

 individuals of the commoner species ; but these are for 

 the most part old and worn-out specimens, and it may be 

 reasonably doubted whether they would Jive long enough to 

 secure the permanence of the species by reproduction at the 

 advent of the following damp, warm season in the month of 

 May. This, on the contrary, probably takes place exclusively 

 or principally by eggs which have been dormant during the 

 dry season, as we may infer from the fact that immediately on 



