COOKE : ON GEOGRAPHICAL DISTEIBUTION. 101 



be that surroundings are produced wliich are wholly unfavourable to 

 the orgauism under consideration — in other words, it may be unable 

 to sustain existence any longer. 



So far as our present knowledge extends, we are unable to 

 determine, with any approach to demonstration, what amount of 

 modification in surroundings becomes unfavourable to the life of 

 a particular species. Changes apparently insignificant on the one 

 side produce, at times, profound modifications on the other, and it is 

 seldom an easy matter to refer with certainty the production of 

 a definite change in form to its cauna cansans, or, conversely, to 

 predict with accuracy what particular modification of form will result 

 from a known environmental change. For instance, specimens of 

 Littorina rudis, Mat., from the coast of Labrador, are habitually much 

 eroded,^ and our common L. obtusata, L., as we follow it northwards 

 in Norway tends more and more to assume the form known as 

 pal/iata, Say ; but no precise explanation of these modifications is 

 forthcoming. Conversely, we cannot predict what particular change 

 of form will occur wlien Limncea pereger is found living in hot 

 water, nor would it be reasonable to assume that all Limnaia living 

 in hot springs were similarly modified. One thing is j)lain, that 

 violent and rapid changes of condition destroy life, while gradual 

 changes are readily tolerated. Even this rule would seem to have its 

 apparent exceptions, for nothing is more striking than to note how 

 certain common littoral marine species begin to die out or become 

 rare on the coasts of South- West Sweden and East Denmark, where 

 the water is not yet brackish. The water of the Kattegat can be but 

 sliglitly affected by the diminished salinity of the Baltic, and yet we 

 find that such species as Purpura lapillus, Patella vuJgata, Ostrea 

 edulifi, and all the littoral Trochidfe, which are entirely wanting in 

 the Baltic, are but feebly represented in that broad strait. 



Science has long been accustomed to distinguish various areas or 

 zones of distribution, the littoral, the laminarian, the nullipore, or 

 coralline, and the benthal, abyssal, or deep - sea zone, each 

 characterized by its own peculiar groups of ilollusca. Scientific 

 ex])editions. from those of the Lightning and Pormpine in 1868-70, 

 and of the Challenger \\\ 1873-6, down to the most recent dredgings 

 of the Prince of ilonaco in the Hirondelle and Princesse Alice in 1912, 

 have established the fact that an increasing number of species are 

 found to live at very distant points on the ocean floor, the uniformity 

 of environment, the absence of sharp breaks in the conditions of life 

 in the great depths, offering only slight barriers to dispersal, and 

 admitting of the widely extended range both of genera and species. 

 Thus Scaphander 2^it7ictostriafus has been found off Spitzbergen, in 

 the West of Ireland, the Azores, and off Culebra Island, West Indies; 

 Philine aperta not only in the seas of Norway, the whole of Western 

 Europe, and the Mediterranean, but also off the Canaries, the Cape 

 Verde, the Cape of Good Hope, East Africa, and the Philippines.- 



1 K. J. Bush, Proc. U.S. Nat. Mus., vol. vi, pp. 236-47. 1883. 



- N. Odbner, Kungl. Svensk. Vetensk. Handl., vol. xli (4), pp. 46, 55, 1907. 



