154 THREE CRUISES OF THE " BLAKE." 



the swimming-bladder precedes in time the lung, which first de- 

 veloped among reptiles. The development is from aquatic ba- 

 trachians to saurian s, and finally to warm-blooded vertebrates, 

 adapted mainly to a terrestrial existence. 



If, as has been suggested by Mosaley, the condition of life 

 of the earliest littoral types was pelagic, we may find in this 

 an explanation of the reversion of the larval forms of so large 

 a number of our littoral species to a pelagic free-swimming 

 stage. 



The believer in cataclysms and the man who seeks in natural 

 causes an explanation of the phenomena around him will each 

 put his own reading on all attempts to explain the causes of the 

 apparent revolutions of earlier geological periods. As those rev- 

 olutions were undoubtedly limited to well defined areas, species 

 living within narrow boundaries in the depths of the ocean may 

 have continued to exist till from local disturbances they in their 

 turn disappeared. 



We are justified in explaining the difference between adjoin- 

 ing deposits, the one barren of animal life, the other crowded 

 with animal remains, by supposing conditions similar to those we 

 find in contiguous areas in the depths of the sea. These differ- 

 ences imply variations in the physical conditions of adjoining 

 areas ; they are not climatic or necessarily due to geological 

 chanofes. One area will be teemino- with animal life, while the 

 other, either from want of food, from the constant deposition of 

 sediment, or from the sudden changes of temperature affecting 

 it, may be a desert on the surface of which no animal life can 

 maintain itself. 



At the time when the greater part of the surface of the earth 

 was covered by water, during the laurentian, huronian, and 

 Cambrian periods, the seas contained annelid tracks, sponges, 

 polyps, some echinoderms, and brachiopods, with few marine 

 plants. From these low types, whether they are the earliest 

 or not, must have descended the present population of the seas 

 and land, both animal and vegetable. The study of the cur- 

 rents of early geological periods can to a certain extent give 

 us a clue to the regions from which subsequent marine faunae 

 have descended, radiating thence little by little over the globe. 



