June, 1891. 
ANIMAL PEDIGREES. 
121 
ANIMAL PEDIGREES. 
BY A. MILNES MARSHALL, M.A., M.D., D.SC., F.R.S., 
BEYER PROFESSOR OF ZOOLOGY IN OWENS COLLEGE, MANCHESTER. 
(Continued from page 101.) 
A very simple example of recapitulation is afforded by the 
eyes of the sole, flounder, plaice, turbot, and their allies. 
These “ flat fish v have their bodies greatly compressed 
laterally, and the two surfaces, really the right and left sides 
of the animal, unlike, one being white or nearly so, and the 
other coloured. The flat fish has two eyes, but these, in 
place of being situated, as in other fish, one on each side of 
the head, are both on the coloured side. The advantage to 
the fish is clear, for a flat fish when at rest lies on the sea 
bottom, with its white surface downwards and the coloured 
one upwards. In such a position an eye situated on the 
white surface could be of no use to the fish, and might even 
become a source of danger, owing to its liability to injury 
from stones or other hard bodies on the sea bottom. 
No one would maintain that flat fish were specially created 
as such. The totality of their organisation shows clearly 
enough that they are true fish, akin to others in which the 
eyes are symmetrically placed one on each side of the head, 
in the position they normally hold among vertebrates. We 
must therefore suppose that flat fish are descended from other 
fish in which the eyes are normally situated. 
The Recapitulation Theory supplies a ready test; and on 
employing it, i.e., on studying the development of the flat fish, 
we obtain a conclusive answer. A voung flounder or other 
flat fish, on leaving the egg, is shaped just as any ordinary 
fish, and has the two eyes placed symmetrically on the right 
and left sides of the head. In Plate VIII., Fig. 5, the young- 
flounder is shown six days after hatching, the eyes being still 
perfectly symmetrical. As the young fish increases in size, 
the shape gradually approaches that of the adult; the body 
increases in height, and becomes flattened laterally, the median, 
dorsal, and ventral fins becoming greatly developed at the same 
time ; and the fish now begins to adopt the habit of the 
adult of lying on one side on the sea bottom. Another change 
occurs : the eye of the side on which the fish lies, the left 
side in a flounder, becomes shifted slightly forwards, then 
rotated on to the top of the head, and finally twisted com¬ 
pletely over to the opposite or right side. In Fig. 6, 
which is drawn to the same scale as Fig. 5, the stage 
is represented in which the left eye is just appearing on the 
top of the head ; while in Fig. 7, which shows the adult 
