644 THE AMERICAN NATURALIST.  [Vow. XXXII. 
the bodies of microscopic animals, or on the eggs and young 
of their own species, some living on the bottom, and others at 
the surface. Of marine plants of the Cambrian there are but 
slight traces, and it is evident that what there were were 
restricted to the coasts and to shallow water. The old idea that 
plants originally served as the basis of animal life must be dis- 
carded. As at present no plant life exists below a few fathoms, 
a hundred perhaps at the most, and since below these limits 
the ocean depths are packed with animal life which exists 
entirely on the young or the adults of weaker forms, so must the 
rise and progress of animal life have been quite independent 
of that of plants. The lowest plants and animals may have 
evolved from some common bit of protoplasm, some protist, 
but the evolution of the animal types became very soon vastly 
more complex. The specialization of parts and adaptation to 
the environment were more thorough and rapid in the lowest 
animals evidently in consequence of the greater power of loco- 
motion, and aggressiveness in obtaining food from living organ- 
isms, and the adaptability of animal life to various oceanic 
conditions, especially temperature, bathymetrical conditions 
and a varying sea bottom. 
This rapid differentiation and multiplication of different fam- 
ily, ordinal, and class ancestral types went on without those 
biological checks which operated in later times, when the seas 
and land masses of the globe became more crowded. There 
was a comparative absence of competition and selection ; this 
being due to the lack of predaceous carnivorous forms to pro- 
duce that balance in nature which afterwards existed. The 
two most successful and abundant types were the trilobites and 
brachiopods ; but the former were not especially aggressive in 
their habits, undoubtedly taking their food in a haphazard way 
by burrowing in the mud or sand, having much the same kind 
of appendages and the same feeding habits as Limulus. The. 
brachiopods were fixed or burrowed in the sand, straining the 
microscopic organisms drawn into the mouth by the currents 
set up through the action of their ciliated arms. The most 
destructive and aggressive Cambrian animals known to us 
were the orthoceratites, but their remains have not yet been 
