No. 431.] PAIRED LIMBS OF THE VERTEBRATES. $ 39 
viewed as nothing more than adaptations to recent physiological 
needs. Thus, as an example, a tangle of nerves and blood ves- 
sels on the tailward side of a fin lobe of a shark may be either 
an important morphological condition as vestige of an ancestral, 
paddle-shaped, ceratodont fin, ¢.g., as Braus maintains, or it 
may be nothing more than a newly developed condition to 
enable muscles and nerves to get in better touch with an 
enlarging and specializing margin of the fin, —which may 
indeed in its growth be developing from stage to stage special 
embryonic and larval movements. Again, the emancipation of 
nerves, vessels, and muscles from the basis of an embryonic 
fin may be the result either of the migration of a gill bar ele- 
ment in the Gegenbaurian sense, or merely of a purely local 
change in the requirements for balancing the constantly grow- 
ing animal. For the center of gravity may shift as the body 
grows, and the fin (in sharks functional even when the “larva " 
is growing within the egg case) may thus have to move forward 
and backward as mechanical needs demand. 
“But,” the Gegenbaurian would maintain, “ our theory is 
supported by fin migration. Of course we are willing to grant 
that the record is obscured by cenogensis, but we have still 
morphological evidence that the ventral fin migrates backward, 
and that the pectoral is relatively stationary, its girdle resem- 
bling the adjacent branchial arches. And we believe, more- 
over, that the mechanical fin needs, which have been noted 
above, are of the utmost importance, since it is in response to 
them that the ventral fin has traveled backward and that the 
pectoral fin has retained, relatively, its primitive position. 
Indeed, the very perfect segmental character of the ventral fin 
can be best explained in terms of a continued hindward migra- 
tion, for the ventral is not the more steadfast and more prim- 
itive fin." 
Now, it has long seemed to me that such a line of argument. 
could be invalidated if it could be shown that during the growth 
of the individual the fins responded to their mechanical require- 
ments in just the opposite way ; that is, that the ventral fin 
Was zot, and that the pectoral fin was, the migrating element. 
And the present paper has grown out of an attempt to trace, 
