172 Muscular Variations 
The Lemurs and the Cebidae continue to perpetuate this type which 
has become modified in Man, the Anthropoids and the Old World mon- 
keys generally by the suppression of the supracondylar process and 
foramen with the consequent alteration in the position of the artery 
and the median nerve, although in these latter forms the primitive 
arrangement may reappear as an individual variation. (Ataval variu- 
tion, vide infra, p. 174.) 
2. The ancestral primate type, in deviating from the common mam- 
mahan stem, had already modified the arrangement of these structures, 
by loss of the supracondyloid foramen. It would then be necessary 
to assume that in their derivation from the primate stem the ancestors 
of the living Cebidae had reverted in this structural detail to the 
common mammalian type and had preserved and transmitted such 
reversion as their descendants further specialized to form the Cebus 
group. (Progonal reversion, vide infra, p. 174.) 
The first hypothesis appears the better founded of the two. Obser- 
vation teaches us that structural differentiation within a_ lhmited 
group is usually progressive. When once carried to the point of mor- 
phological independence further special modifications may take place, 
but return to the primitive condition for an entire group of individu- 
als composing the species or genus is the less warrantable supposition. 
Examples are encountered in which a partially degenerate and rudi- 
mentary structure has revived under the influence of a new functional 
adaptation. Moreover, under adequate physiological stimulus, an organ 
may even be evolved de novo, along the same paths and following the 
same developmental lines which far back in the phylogenetic history 
of the species led to the production of its prototype, which has, in 
course of the intervening evolutionary period, become rudimentary or 
adapted to other functional purposes. ‘hus in the Teleost families 
of the Ophiocephalidae and Labyrinthict the pharyngeal mucous mem- 
brane is in the process of evolving the vascular area, serving purposes 
of respiration, which we must regard as the phylogenetic point of 
departure for the evolution of the swim-bladder, an organ, present in 
both of these 'Teleost groups, which has long lost its primary respiratory 
character and has been adapted to other uses. Yet under the same 
stimulus of physiological environment, furnished by the shallow and 
muddy waters of the habitat of these forms, the same evolutionary 
structural development is beginning over again. Such conditions are, 
however, exceptional and depend upon special physiological demands. 
It is more in conformity with the results of our observations to consider, 
in the example in hand, the supracondylar foramen as a character 
