I THE INTRODUCTION OF NEW SPECIES 19 
not materially opposed to any of them. It also claims a 
superiority over previous hypotheses, on the ground that it 
not merely explains, but necessitates what exists. Granted 
the law, and many of the most important facts in Nature 
could not have been otherwise, but are almost as necessary 
deductions from it as are the elliptic orbits of the planets 
from the law of gravitation. 
