SALPA IN RELATION TO EVOLUTION OF LIFE. 165 



delicate and sensitive animals, and we had no difficulty in keeping 

 alive, in water taken from the sounds, the surface animals which 

 we caught outside. Even trachomedusse and doliolums, which are 

 extremely sensitive to impurities in the water, could be kept alive 

 in the house very much better than in any other place where I 

 have ever tried to keep them, and instead of being injurious, the 

 pure water of the sounds is peculiarly favorable for use in aquaria 

 for surface animals. 



The scarcity of floating organisms can have only one explana- 

 tion. They are eaten up, and competition for food is so fierce that 

 nearly every organism which is swept in by the tide, and nearly 

 every larva which is born in the sounds, is snatched by the tenta- 

 cles around some hungry mouth. 



Nothing could illustrate the fierceness of the struggle for food 

 among the animals on a crowded sea-bottom more vividly than 

 the emptiness of the water in coral sounds. The only larvae 

 which have much chance of establishing themselves for life are 

 those which are so fortunate as to be swept out into the open 

 ocean, where they can complete their larval life under the milder 

 competition of the pelagic fauna, and while it is usually stated 

 that the pelagic habit has been retained by the larvae of bottom ani- 

 mals for the purpose of distributing the species, it is more probable 

 that it has been retained on account of its comparative safety. 



There can be no doubt, in view of these facts, that competition 

 came swiftly after the establishment of the first bottom fauna, and 

 that it soon became very rigorous and led to rapid evolution ; and 

 we must also remember that life on the bottom introduced many 

 new opportunities for divergent modification and for the perfect- 

 ing of animals. 



The increase in size, which came with the economy of energy, 

 increased the possibilities of variation, and led to the natural selec- 

 tion of those peculiarities which improved the efficiency of various 

 parts of the body in their functions of relations to each other, and 

 this has certainly been an important factor in the evolution ot 

 complicated organisms. 



The new mode of life also permitted the acquisition of protec- 

 tive shells, hard supporting skeletons, and other imperishable 

 structures, and it is therefore probable that the history of evolu- 



