LIFE AT THE SURFACE OF THE SEA. 195 
estimated that in some parts of the Baltic there are 
140,000,000 microscopic plants in every ten cubic metres of 
water. In the Atlantic the Challenger passed for days 
through water full of minute Algz (Trichodesmium), gleaming 
in the water like particles of mica! Off our own coasts 
these minute forms may be so abundant as to coat the 
fishermen’s nets with an odoriferous layer. This plant life 
is especially rich in April and May, just when the. young 
post-larval fishes are most abundant in inshore waters. 
Immense numbers of Foraminifera and Radiolaria feed on the 
Diatoms, and form food for the smaller Crustacea, and for the 
smaller larval fishes. 
This wonderful abundance of a lowly form of plant life is 
a fact of fundamental importance in the history of life in the 
waters of our globe. But for this all other life would cease, 
and the sea would be an uninhabited waste. Animals have 
no power of maintaining themselves upon ar inorganic diet, 
and obviously could not go on living upon each other 
indefinitely. This abundant plant life is believed to be 
confined to a comparatively shallow surface zone. Sunlight 
penetrates a comparatively short distance through the water, 
and although a very small amount of light suffices for the 
production of chlorophyll, Regnard, in some careful experi- 
ments, could find no evidence of the breaking up of CO, 
by chlorophyll at a depth of 10 metres. 
It has, however, been suggested that even at considerable 
depths some Algz might continue to live by the light given 
off from phosphorescent animals. The animals belonging to 
the surface fauna no doubt continue abundant at consider- 
ably greater depths than those in which plants are able to 
flourish, 
THE ORIGIN OF THE PELAGIC FAUNA. 
The consideration of these simple living beings leads 
us on to another question—the origin of the fauna of the 
surface. Whence came this wondrously rich and diverse 
life? Most naturalists now believe that it is at least highly 
probable that life originated in the sea, and the simple single- 
celled plants and animals of which I have just been speaking 
carry us back in imagination to an immensely distant past, 
