VIII. 



The Evolution of life. 1 



PROFESSOR BROOKS has printed in advance from his memoir 

 on the genus Salpa, chap, vii., on " Salpa in its Relation to the 

 Evolution of Life," and chap, viii., on the " Origin of the Chordata 

 considered in its Relation to Pelagic Influences." Salpa is dis- 

 tinctively a pelagic animal borne hither and thither on the surface of 

 the ocean as currents and the wind direct, living entirely on the 

 minute micro-organisms and, unlike so many of its nearest relations, 

 spending no part of its life attached to rocks, or weeds, or mud. 

 Professor Brooks has been studying its life-history and its structure 

 minutely, and has come to some biological conclusions of very general 

 interest, conclusions which, perhaps unnecessarily, he apologises for 

 as heterodox ; but we hope that morphology so far is free from the 

 doctrine of authority. Its generalisations tread so closely on the 

 heels of observed facts that, granted the necessary skill in manipulation, 

 no morphologist need accept much on the authority of a memoir, or a 

 text-book, that he cannot verify with the scalpel and microscope. 



Dr. Brooks leads up to his results by consideration of the contrast 

 between terrestrial life and marine life. The surface of the land is 

 more or less completely covered by verdure. Forest trees, and herbs 

 and grasses, ferns and mosses stretch their green expanses into the 

 air, and the chlorophyll of the cells in the presence of t sunlight is 

 constantly building up from the inorganic elements of the earth and 

 air rich supplies of protoplasmic food ; and so on the surface of the 

 earth the vegetable eater, as insect, or bird, or mammal, abounds 

 not only in species but in individuals, while carnivorous forms are 

 found in smaller numbers, and obviously in immediate dependence 

 on the vegetarian world. 



The ocean waste is very different. Here and there are floating 

 tufts of sargassum, but save round the coasts the vegetable world is 

 meagre, and the richly-coloured gardens of the bottom are nearly all 

 animal forms. So, too, in the sea, the conspicuous animals are 

 nearly all carnivorous. The seals live upon fish ; the sea elephants 

 and walruses on lamellibranchs ; whales, dolphins, and porpoises 



1 Studies from the Biological Laboratory of Johns Hopkins University. 

 vol. v.. No. 3. "Salpa in its Relation to the Evolution of Life." By Professor 

 W. K. Brooks, Ph.D., Baltimore, May, 1893. 



