Geo. 8S. Huntington ~ 173 
transmitted from the common mammalian stem to the Primate ances- 
tral line, just as it has been transmitted to the branches of other mam- 
malian orders. The Lemurs, Cebidae and allied genera among living 
Primates have retained this structural character. It has disappeared 
in the Old World forms of the order, as a normal occurrence, but 
appears as a reversional variation in individuals, indicating their deriva- 
tion from the common primate ancestral type possessing it. 
It will be appreciated that in many instances close scrutiny “is 
required in order to correctly determine the phylogenetic value of any 
given structure. To illustrate, for a moment, by another example 
taken from the same group. The Cebide are uniformly distinguished 
by a partly convoluted cecum of comparatively narrow lumen, placed 
laterad to the continuous line of the ileo-colic junction, differing 
markedly from the form and position of the cecal pouch in all other 
Primates, except the Marmosets, which somewhat resemble the Cebidx 
in the arrangement and structure of the cecum. Moreover, the Cebide 
in this particular respect resemble only one other group among Mam- 
malia in general, namely the Cynoid carnivora. Here we are evidently 
dealing with secondary modifications and adaptations of a preceding 
common mammalian, or even generalized vertebrate type, which in the 
Cebidz and Cynoidea have taken parallel lines of development and have 
led independently of each other to the production of the coiled lateral 
pouch characteristic of these forms. More complete and detailed knowl- 
edge of comparative anatomy will unquestionably furnish important 
indications as to the relative point of derivation of the individual genera 
and species from the common order-stem. 
To return to the subject of muscular homologies we may assume that 
the common prototype of the order will in general present simpler 
structural conditions, with less complete segmentation of the entire 
mass into smaller subdivisions or individual muscles, and with fewer and 
more generalized points of skeletal attachment. 
Reversional muscular variations may be grouped in accordance with 
their probable phylogenetic significance, in reference to their point of 
derivation from the stem, which has led by gradual differentiation to 
the development of the specific form of the muscle or muscle-group 
normal for the type in question. (Pl. II, Fig. 2.) 
ARCHEAL REVERSIONAL VARIATIONS. —A limited number of muscu- 
lar variations repeat conditions which are not normally encountered in 
°"Apyy = beginning, origin. 
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