196 DR W. B. DRUMMOND ON 
when they, or simple things like them, alone constituted the 
fauna of the sea. For the maintenance of such a fauna the 
sea is peculiarly favourable. The land surface of the earth, 
with its extreme diversity of soil, of altitude, of climate, of 
heat and cold, of rain and drought, presents all the condi- 
tions around which, in their struggle for existence, animals 
and plants have slowly diverged through successive 
modifications and adaptations into the varied fauna and 
flora that exists to-day. The struggle for existence has 
been a struggle not only with competitors for place, and 
food, and air, and light, but, in a different sense, with the 
inorganic surroundings which in their diversity have allowed 
the most diverse modifications to find a place for growth 
and development, 
In the sea, however, the inorganic surroundings, instead 
of being strikingly diverse, are strikingly uniform. The 
water of the sea, with its abundant saline constituents, is 
practically a nutritive medium containing all the elements 
necessary for the existence of plant life. All day the sur- 
face waters are exposed to the sunlight. But a little below 
the surface the stormiest winds do not disturb the prevail- 
ing calm. The temperature is remarkably uniform over 
enormous areas. The marine currents carry the Planktonic 
forms about, without their being otherwise affected than if 
they were in perfectly still water. Moreover, in the sea 
there is an absence of those natural barriers which, on the 
land surface, favour the segregation of diverging species. 
A fauna, therefore, composed of single cells would in the 
sea be practically free from all those conditions under which 
the fauna of the land has developed into its present form ; 
and such cells, growing in bulk in a medium which bathed 
them on all sides, would evidently find the most favourable 
conditions for their continued increase by parting the one 
part from the other every time they underwent division. 
The reason for the appearance of the many-celled from the 
single-celled forms has been a fruitful subject for discussion. 
Theoretically, at any rate, it would seem that the answer 
is not to be found in the surface waters of the sea. Let us 
then for a little leave these simple forms alone and consider 
again the surface fauna of our own day. 
