CHAPTER VII. 

 SALPA IN ITS RELATION TO THE EVOLUTION OF LIFE. 



Salpa is distinctively a pelagic animal, adapted by its whole structure 

 for a free existence, and for life at the expense of the micro-organisms in 

 the water of the ocean. 



To understand its position and significance in the economy of nature, 

 we must have before us the broad outlines, at least, of a picture of the 

 conditions under which oceanic life has been evolved. 



I believe that the history of the evolution of Salpa, as told by its 

 embryology, is most suggestive and important, and that it contributes to 

 the solution of some of the most profound and fundamental problems of 

 biology, and brings us into conflict with some of the most favorite dicta 

 of modern morphology. I shall therefore devote considerable space to a 

 review of certain familiar features of ocean life, in order that I may pre- 

 sent in this way my view of the significance of the phylogeny of Salpa, 

 in its bearing upon the first principles of morphology. 



Contrast between Terrestrial Life and Marine Life. 



In a picture of the land, the mind calls up a vast expanse of verdure, 

 broken only by water, and stretching through forest and meadow from 

 high up on the mountains, over hills and valleys and plains, down to 

 the sea. 



Our picture of the ocean is an empty waste, stretching on and on 

 with no break in the monotony, except, at long intervals, a floating tuft 

 of sargassum, or a flying fish, or a wandering sea-bird, and we never 

 think of the ocean as the home of vegetable life. 



It contains plant-like animals, "zoophytes," in abundance, but while 

 they resemble plants or flowers in form and color, and in their mode of 

 growth, they are true animals and not plants. 



At Nassau, in the Bahama Islands, the visitor is taken in a small 

 boat, with windows of plate glass set in the bottom, to visit the "sea- 

 gardens" at the inner end of a channel, through which the pure water 

 from the open sea flows between two coral islands, into the lagoon. 



