Note on the Pectoral Fin of the Sole, Achirus capensis. 
10 1 
We may assume that the altered position of the body of the flat fish 
was not assumed suddenly and by all its members at once, and that it came 
about probably in correlation with a changing external environment. A 
change in environment, brought about primarily by a simple change in 
physical conditions, may be followed by much more complex changes in the 
actual surroundings of the organism. By way of illustration we may 
suppose that the first change was a purely physical one, as, for instance, the 
gradual deposition of mud. Secondary changes would follow, the mud 
becoming populated by a variety of its characteristic forms of life. These 
forms in their turn would attract other predatory fish, etc., which feed on 
them, and so a great many new factors — these and others — would arise 
which could possibly lead to changes in the ancestral sole. One possibility is 
suggested by an observation on a particular species of fish, the blenny, Clinus, 
of which there is a great variety at the Cape. It is active in its habits, 
and preys on other small fish, crabs, etc. Its eyes are well developed, and 
resemble the motile eyes of the chameleon. In captivity it was seen 
frequently to seize and devour small mullet swimming above it at the 
surface, and it was observed that, in order to watch its prey more readily, it 
frequently lay over on one side, thus directing an eye upwards. It can be 
readily concluded that if this device is adopted for detecting prey, it will 
serve equally well for detecting enemies, and may have been adopted by flat 
fishes ; another possible factor is the changed source and changed kind of 
food, consisting of small animals in the muddy ground. The reason for the 
change of position need not, however, be considered here, and these possible 
factors are merely mentioned to indicate how complex the external environ- 
ment may become. The suggested change in environment by a gradual 
deposition of mud is also purely illustrative. A change in environment 
may be brought about in other simple ways, as, for instance, by gradual 
migration of the animal itself to other localities. 
With the change from a vertical to a horizontal position the function of 
the pectoral fin of maintaining the body in a vertical position would be lost, 
and, owing to disuse, it would tend to diminish in size. This diminution in 
size, accompanied probably by more complex physiological changes, would 
of course appear in the succeeding generation if the identical factors which 
brought it about in the parent were present, and it follows that if these were 
more pronounced, as they would be in a changing environment, the effects 
would be more marked. This process of diminution would be arrested at 
any stage, in which it began to function in another capacity, as has occurred 
in the Achirus. 
It might be suggested that the reduction of the pectoral fin is in co-ordi- 
nation with the elongation of the body as happens in the case of many fish. 
This, however, is not the case in Achirus, where the body is not elongate, but 
rather short in proportion to its depth. 
