14 ON THE LAW WHICH HAS REGULATED 
©. Lyell in his admirable “ Principles.” Geological 
changes, however gradual, must occasionally have 
modified external conditions to such an extent as 
to have rendered the existence of certain species 
impossible. The extinction would in most cases be 
effected by a gradual dying-out, but in some in- 
stances there might have been a sudden destruction 
of a species of limited range. To discover how the 
extinct speciés have from time to time been replaced 
by new ones down to the very latest geological period, 
is the most difficult, and at the same time the most 
interesting problem in the natural history of the 
earth. The present inquiry, which seeks to elimi- 
nate from known facts a law which has determined, 
to a certain degree, what species could and did ap- 
pear at a given epoch, may, it is hoped, be consi- 
dered as one step in the right direction towards a 
complete solution of it. 
High Organization of very ancient Animals consistent 
with this Law. 
Much discussion has of late years taken place on 
the question, whether the succession of life upon the 
globe has been from a lower to a higher degree of 
organization. The admitted facts seem to show that 
there has been a general, but not a detailed pro- 
gression. Mollusca and Radiata existed before Ver- 
tebrata, and the progression from Fishes to Reptiles 
and Mammalia, and also from the lower mammals 
to the higher, is indisputable. On the other hand, 
