464 BUMPUS. [Vor. XIL. 
region, and thus about twelve specimens should have sacral 
ribs on the XXI or XXII vertebra. Such a disposition of the 
sacral ribs does not occur, though its absence may be attributed 
to a presumed tendency of the parent form towards “ fixity” 
or ‘specific stability.” 
Rejecting the theory of atavistic variation, I think the for- 
ward homeeosis of the sacral vertebra may be explained on 
other grounds. 
In the first place, the origins of the processes that bear the 
ribs, as will be noted by reference to Pl. A, Spec. 87, arise not 
from the middle, but from the posterior end of each vertebra, 
making it a much shorter distance from the transverse process 
of the XIX to the territory of the XX, than from the trans- 
verse process of the XIX to the territory of the XVIII trunk 
metamere. It will thus seem more probable that variations 
occurring in the course of ontogenetic development will fall on 
the side of nearer proximity ; and the Anlage of the limb, com- 
pounded though it may be by contributions from several trunk 
segments, will depart less from its normal position when it 
finally falls within the territory of the XX than when it falls 
within the territory of the XVIII vertebral segment. This 
explanation rests upon the assumption that the territory of 
the XX metamere once invaded, the secondary processes of 
growth within that segment will arrange for the accommoda- 
tion of the sacral rib at its normal place, even though its 
Anlage was first located in the anterior portion of the segment. 
A third and possibly valid explanation is offered by anatomy 
alone. The XVIII, XIX, and XX segments are not, in the 
adult at least, of equal length. The Anlage of the limb would 
be obliged to vary considerably more, in a linear direction, in 
order to influence the XVIII than it would to influence the 
XX vertebral segment. 
Though these two possible explanations of the more fre- 
quent occurrence of forward homceosis are here given, they 
are both dependent upon an assumed variation in the position 
of the Anlage of the appendage with respect to some one 
1 The occurrence in certain fishes of pelvic fins, which lie in front of the pecto- 
ral fins, is in this connection of especial interest. 
