THREE CRUISES OF THE “ BLAKE.” 
154 
the swimming-bladder precedes in time the lung, which first de- 
veloped among reptiles. The development is from aquatic ba- 
trachians to saurians, and finally to warm-blooded vertebrates, 
adapted mainly to a terrestrial existence. 
If, as has been suggested by Moseley, 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 Svoe-awimming 
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 
changes. One area will be teeming 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 faunæ 
have descended, radiating thence little by little over the globe. 
