19^0. CoLGAN. — The Adaptability of Littoral Molhisca. 131 



Percentage Survival, of Purpura lapillus from high-water and from 

 low-water mark and oi Littorina ritdts from high-water and from half- 

 tide mark after immersion for equal periods in fresh-water and in 

 water of treble the salinity of sea- water. 



To sum up, the net result of these observations and ex- 

 periments is to show that the high-water individuals of these 

 littoral species have a greater power of endtiring exposure to 

 air and immersion in fresh water than the low-water or lialf- 

 tide individuals, while they have a lesser power of enduring 

 immersion in water of abnormal salinit3^ There is every 

 reason to suppose that in the Q2i^^oi Purpia-a lapilhis, at least 

 a fuller course of experiment would demonstrate for a half- 

 tide group a stage of adaptabilit}- intermediate between those 

 of the high and low-level groups. This species is to a great 

 extent sedentary. The high-level individuals are, no doubt, 

 hatched a little below high-water mark, and pass their lives in 

 this upper zone, as the half-tide and low-tide individuals are 

 born and pass their lives in the middle and lower tidal zones; 

 and the grades of adaptability shown by the inhabitants of 

 each zone probably point to the progress of a slow evolution 

 from a marine to a terrestrial existence. 



Two questions are suggested by the results set out in the 

 preceding pages. First, are the tests to which the littoral 

 species were subjected during the course of these experiments 

 of greater or less severity than the species are reqtiired to 

 withstand in a state of nature ? Second, what are the pecu- 

 liarities of structure which in each species may be held to 

 favour or to stand in the way of its attaining to a high degree 

 of adaptability to abnormal conditions. 



Taking the second of these questions first, it would appear 

 that the conditions favouring a high degree ot adaptability are 



