456 TRANSACTIONS OF THE AMERICAN PHILOSOPHICAL SOCIETY 



2. Changeableness, initiated perhaps by progressive breaking down of very complex 



molecules, but constantly checked or controlled by 



(a) viability (negative selection); 



(b) usefulness (positive selection); 



3. Multiplicity of each species: the power to engender innumerable descendants en- 



dowed with hereditary advantages gained in the past; 



4. Time — the four hundred-odd million years of fish history. 



