Q24 COMPARATIVE ZOOLOGY. 
prevailing characters which are the most constant.” And 
such a classification can not be linear. It is impossible to 
arrange all animal forms from the Sponge to Man in a 
single line, like the steps of a ladder, according to rank. 
Nature passes in so many ways from one type to another, 
and so multiplied are the relations between animals, that 
one series is out of the question. ‘There is a number of 
series, and series within series, sometimes proceeding in 
parallel lines, but more often divergent. The animals ar- 
range themselves in radiating groups, each group being 
connected, not with two groups merely, one above and the 
other below, but with several. Life has been likened to a 
great tree with countless branches spreading widely from 
a common trunk, and deriving their origin from a com- 
mon root; branches bearing all manner of flowers, every 
fashion of leaves, and all kinds of fruit, and these for ev- 
ery use. 
The groups into which we are able to cast the various 
forms of animal development are very unequal and dis- 
similar. We must remember that a genus, order, or class 
is not of equal value throughout the kngdom. Moreover, 
each division is allied to others in different degrees—the 
distance between any two being the measure of that affin- 
ity. The lines between some are sharp and clear, between 
others indefinite. Like the islands of an archipelago, some 
groups merge into one another through connecting reefs, 
others are sharply separated by unfathomable seas, yet all 
have one common basis. Links have been found reveal- 
ing a relationship, near or distant, even between animals 
whose forms are very unlike. How different a Fish and 
a Lizard! yet there is a strange creature (the Axolotl of 
Mexico) whose organization is intermediate between the 
two, so that it is difficult to determine to which group it 
belongs. The Slow-worm is a transition between Lizards 
and Snakes; while the extinct Ichthyosaurus bridges the 
