CLASSIFICATION 7 
enormous structural and physiological changes had of 
necessity to have been attained. The frame of the 
head and trunk has become moulded as in the fish’s 
form, contours have been elaborately filled out and 
rounded, median dermal keels developed, vein valves lost, 
and the legs transformed into fin-like appendages. 
The form of the fish is accordingly to be looked upon as 
cast in a more or less common mould by its environment. 
Its internal structures, as in the cetacean, are also ob- 
served to be modified in accordance with its external form. 
This is a factor in the evolution of fishes which appears 
in every group and sub-group. And it has ever stood in 
the way of classifying them satisfactorily according to 
their kinships. 
“Fishes,” used as a popular term, may include Lam- 
preys, Sharks, Chimzeroids, Lung-fishes, and “Modern 
Fishes” (Teleostomes),—the major groups to be dis- 
cussed in the present book. But the relative position of 
each of these divisions must at present remain more or 
less doubtful. The group of the Lampreys is certainly 
widely removed from the remaining ones, standing mid- 
way between the simplest chordate, Amphioxus, and the 
true fishes: it is usually given a rank co-ordinate with 
either of these, and, in fact, with all other groups 
of vertebrates, taken collectively. Sharks, Chimezeroids, 
Teleostomes, may be taken to represent true fishes ; and 
each might be assigned co-ordinate rank, although geneti- 
cally the Chimezroids are certainly far more closely allied 
to the Sharks than are the Teleostomes. The Lung-fishes, 
as a widely divergent group, appear, as W. N. Parker has 
suggested, to be reasonably entitled to a rank equivalent 
to that of the three groups of true fishes taken together. 
