SALPA IN RELATION TO EVOLUTION OF LIFE. 161 



wards swept away, like clouds before the wind, by geological changes, 

 and that the bottom fauna which we know was not the first. 



Colonies which started in shallow water were exposed to accidents 

 from which those in great depths were free, and in view of our 

 present knowledge of the permanency of the sea-floor and of the 

 broad outlines of the continents, it is not impossible that the first 

 fauna which settled in the deep zone around the continents may 

 have persisted and given rise to our modern life. However this 

 may be, we must regard this deep zone as the birthplace of the 

 fauna which has survived ; as the ancestral home of all the im- 

 proved metazoa. 



The effect of life upon the bottom is more interesting than the 

 place where it began, and we have now to consider its influence in 

 the evolution of animals. 



The effect of the secondary acquisition of a sedentary life by 

 modern animals has been fully discussed by many writers, but no 

 one, so far as I am aware, has ever considered the effect of the first 

 settlement of the bottom by pelagic animals, all whose competitors 

 and enemies had previously been pelagic. 



It is doubtful whether the animals which first settled on the 

 bottom secured any more food than the floating ones, but they un- 

 doubtedly obtained it with less effort, and were able to devote their 

 superfluous energy to growth and to multiplication, and thus to 

 become larger and to increase in numbers faster than pelagic animals. 



Their sedentary life must have been favorable to both sexual and 

 asexual multiplication, and the tendency to multiply by budding 

 must have been quickly rendered more active. It is sometimes 

 stated that the capacity for budding has been acquired among the 

 metazoa as the result of a sedentary life, but this view hardly seems 

 to be the true one. Capacity for asexual multiplication is very old, 

 older in all probability than sexual reproduction, and there is no 

 reason to believe that it has ever been lost even by the highest 

 animals, for it must be regarded as nothing more, in ultimate 

 analysis, than discontinuous growth. The tissues of all animals 

 have vegetative power, and external influences determine whether 

 this shall result in continuous or discontinuous growth, and a trace 

 of the power to multiply asexually is retained even among the em- 

 bryos of mammals. It is therefore wrong to speak of the acquisition 

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