Geo. 8. Huntington 1! 
. 
panniculus or of the primitive continuous plane of Pectoralis and Latis- 
simus across the axillary space, as a normal structure in man, although 
occurring in him as a frequent variation. 
In utilizing the results of extensive comparisons for the determination 
of the phylogenetic value and significance of variations three facts are 
found to have an important bearing on these problems: 
1. Within a single mammalian order certain groups are frequently 
sharply differentiated by the uniform development of a structural char- 
acter which is as consistently wanting in other genera and species com- 
posing the order. This is true not only of muscles, but also of morpho- 
logical details in other organic systems. 
2. Such isolated characters, while absent in other species of the order, 
may appear more or less widely distributed in other mammalian orders, 
or even in other vertebrate classes. 
3. Further, they may appear, as variations, in individuals of a species 
which does not normally possess them. 
Thus, for example, among the Primates certain groups, like the 
Lemurs and the Cebide, are characterized by the uniform development 
of the supracondyloid foramen, with the corresponding arrangement 
of the artery, median nerve and Pronator radii teres muscle, while in 
Man, the Anthropoids and in general in the monkeys of the Old World 
the humerus does not normally carry the foramen. 
Now this collection of anatomical details, confined among the Pri- 
mates to certain groups and normally absent in the other genera, although 
observed in them occasionally in individual instances as a variation, pos- 
sesses a very definite structural character and identity, and appears 
widely distributed in other Mammalian orders, such as the Insectivora, 
except Hrinaceus, the Edentates, Marsupials, Monotremes, the Felide 
among Carnivora, etc. In judging of the phylogenetic value of this 
condition in its more isolated appearance in certain specialized primate 
eroups, the following considerations suggest themselves: 
The prevalence of this arrangement in other mammalian orders, not 
to speak of reptilian homologies, indicates clearly that it belongs to the 
cominon mammalian prototype. In all probability it leads back beyond 
the class distinction into the common vertebrate stem (Pl. VII, Fig. 11). 
As regards its persistence in certain primate groups, as the Lemurs 
and the Cebidae, and its default in other genera of the same order, 
two possibilities may be considered: 
1. These structural characters were transmitted from the common 
mammalian stem through the line of Primate adaptation to the com- 
mon primate ancestral type. 
