312 THE HISTORY OF CREATION. 
animals—in the different classes, families, and species of it 
—have unequally developed, differentiated, and perfected 
themselves. It shows us how far the succession of classes 
of vertebrate animals, from the Fishes upwards, through the 
Amphibia to the Mammals, and here again, from the 
lower to the higher orders of Mammals, forms a progressive 
series or ladder. This attempt to establish a connected 
anatomical developmental series we may discover in the 
works of the great comparative anatomists of all ages— - 
in the works of Goethe, Meckel, Cuvier, Johannes Miller, 
Gegenbaur, and Huxley. 
The developmental series of mature forms, which com- 
parative anatomy points out in the different diverging and 
ascending steps of the organic system, and which we call 
the systematic developmental series, is parallel to the 
palzeontological developmental series, because it deals with 
the result of palzontolgical development, and it is parallel 
to the individual developmental series, because this is 
parallel to the palzeontological series. If two parallels are 
parallel to a third, they must be parallel to one another. 
The varied differentiation, and the unequal degree of per- 
fecting which comparative anatomy points out in the 
developmental series of the System, is chiefly determined 
by the ever increasing variety of conditions of existence to 
which the different groups adapt themselves in the struggle 
for life, and by the different degrees of rapidity and com- 
pleteness with which this adaptation has been effected. 
Conservative groups which have retained their inherited 
peculiarities most tenaciously remain, in consequence, at the 
lowest and rudest stage of development. Those groups pro- 
gressing most rapidly and variously, and which have adapted 
