Note on Pelagic Organisms and Evolution. 241 



XXVI. — Note on Pelagic Organisms and Evolution. 

 By J. Graham Kerr. 



(Received 27tli November 1911. Read 27th November 1911.) 



We may probably accept as a general principle that evolutionary change 

 is to a great extent a function 1 of environmental change. So long as the 

 environment remains constant evolution will consist in better and better 

 adaptation to that environment, and no fresh evolutionary path will be 

 struck out. 



This principle is of importance when applied to animals which lead 

 a pelagic existence, for a pelagic environment is probably the type of 

 environment which shows less change than any other. 



Change of environment may be either of two kinds ; it may be temporal 

 or spatial. It may consist in actual change, during the course of time, in 

 the environmental conditions (e.g. geological, climatic, bionomical) of a given 

 area. On the other hand, it may consist simply in the transference of the 

 organism in space from one set of environmental conditions to another, 

 e.g. change from a terrestrial to an arboreal habit, from life in forest to life 

 in open plains, from life in salt water to life in fresh water, and so on. Now 

 it is clear that with regard to each of these two possible types of 

 environmental change the waters of the ocean away from land and 

 away from the bottom form an environment of extraordinary constancy. 

 Organisms inhabiting them have probably less chance than any others 

 of becoming subjected to changed sets of conditions. It follows then, if 

 the principle enunciated above be a correct one, that pelagic organisms will 

 on the whole tend to evolve merely towards a condition of better and 

 better adaptation to the pelagic environment and that the probabilities 

 are against their striking out new lines of descent. 



On such gruunds it seems to me essential to exercise the greatest caution 

 in suggesting or in accepting as probable any hypothetical line of descent for 

 an existing group of organisms which derives them from a pelagic ancestor. 



A like caution is demanded in interpreting resemblances between pelagic 

 organisms, more especially pelagic larvas, as expressions of genetic affinity 

 rather than of adaptation to the same set of environmental conditions. 

 Perhaps the need for such caution may be best illustrated by considering a 

 few of the lines along which, on the principle of adaptation, we might expect, 

 a priori, pelagic larvae to evolve. 



1 I use the word in its mathematical sense. 



