NOTES. 459. 



her disposal an all-sufficient means of reducing the too exuberant mul- 

 tiplication of all very fertile species, in epidemic diseases, and no com- 

 petition is needed or available here.' Now, it seems to me that this 

 sentence can have no other meaning than that I have attributed to it ; 

 according to Wagner the ' struggle for existence ' means nothing else 

 than a competition between two animals for a certain possession. But 

 is there no straggle for existence when a snail endeavours to escape the 

 causes which produce an epidemic ? . Epidemics among land-snails are 

 commonly caused by too great moisture or drought ; those that cannot 

 escape rapidly enough perish ; those that cannot endure drought are 

 destroyed. In an epidemic of rot, or rather of saturation, the old and 

 feeble individuals will perish first ; parching heat is least endurable to 

 the young animals, as their shell and diaphragm are not thick enough to 

 protect them against desiccation. Nay, even a direct struggle is not 

 always entirely avoided. In order to escape from drought, many land- 

 mollusca creep into cracks and fissures in the recks; the first-comers are 

 the best off, for they can creep furthest in, and those that come after 

 close up the opening and prevent the escape of the moisture. Thus, 

 during the dry season in the countries of tlje Mediterranean, for 

 instance, we find the outer rows of snails almost invariably dead, while 

 any considerable number of living ones are only found at some depth. 

 The conditions are reversed when the rainy season comes on. All the 

 rifts and crevices are filled with water ; those lowest down are the first 

 to be immersed, and strive to escape the too abundant supply that soaks 

 tiieir skin ; but the dead shells remain attached above them, or those still 

 living, but not yet aroused by the wet, hiifder them from creeping out, the 

 water penetrates their pores, and in a few hours they are so ' water-logged ' 

 and dropsical as to be incapable of any rapid movement. (It is a great 

 error to suppose that a snail cannot have too much moisture ; if one 

 is plunged into water and prevented from escaping within twenty-four 

 hours, it is so completely sodden as to be quite incapable of crawling.) 

 Is not this a struggle for existence ? It seems to me that it is a very 

 obstinate struggle for existence when one snail, even after its death, can 

 bar the road to life and freedom to one of its companions. - But many 

 of the 'premisses in the passage above quoted from Wagner are false or 

 quite unfounded. He says that Aoliatinella is one of the very fecund 

 species of which the overwhelming multiplication is more effectually 

 hindered by epidemics than by competition or rivalry. This is either 

 false or devoid of foundation ; Acliatinella is oviparous, and produces 

 only a few" young at a time ; how often in the year is perfectly unknown. 

 The assertion that every mature hermaphrodite individual is always ready 

 to pair is certainly not proved. It has never been actually disproved 

 by observation that many snails die without pairing from antipathy, 

 though fully grown and mature, and the extraordinary convolutions and 

 gymnastics performed by snails before pairing lead to the conclusion 



