174 Muscular Variations 
any mammalian type, but which appear homologous with normal mus- 
cles in the lower vertebrate classes. ‘They indicate a return in the devel- 
opment of the individual muscle to a phylogenetic point antedating the 
class-distinctions, a reversion to the common antecedent vertebrate 
myo-type. They illustrate the persistence of structural conditions 
which have not been carried normally into the mammalian stem, but 
which appear in the Sauropsid derivatives of the common vertebrate 
trunk. | 
Proconant REveRSIONAL VARIATIONS.—A muscular variation not 
represented by a homologous muscle normally present in any species of 
the order indicates a reversion to a phylogenetic point preceding the 
derivation of the ancestral order-line, namely to the common antecedent 
mammalian class-stem. 
I have in a former publication * defined these reversions as “ myo- 
typical,” with the desire to emphasize their relation to the “ myo-type” 
of the entire mammalian stem. But it appears preferable to co-ordi- 
nate the terminology, and I therefore propose to designate these varia- 
tions as “ progonal.” 
ATAVAL REVERSIONAL VARIATIONS.—Under this head are to be 
grouped variations reproducing myological characters which are abnor- 
mal for the species in question, but which appear normally in other 
allied species of the same order. They revert to the common ancestral 
myo-type of the order, from which by successive differentiations the 
generic and specific forms included in the order are derived. 
It is of course possible that certain of these ataval variations should 
at the same time be progonal in character, i. e., that the homologous 
muscles should normally appear also in other mammalian orders. 
Careful determination will be required in any given case to assign 
the true phylogenetic value of such variants. 
EXPLANATION OF FIGURES ON PLATES I To VII. 
Fic. 1. Adult human subject. Pectoralis major with deficiency of sterno- 
costal portion, and resulting production of an atypical displaced interme- 
diate Pectoral muscle (Tensor semivaginae articulationis humero-seapularis, 
Gruber, Pectoralis minimus). From a fresh dissection. 
3 tpoyovoc = earlier born, hence an ancestor, used of a god as father of a race, as 
Zev rpdyove (Euripides). 
4« The Significance of Muscular Variations, Illustrated by Reversions of the Anti- 
brachial Flexor Group.” Trans. N. Y. Acad. Sci., Vol. XIV, 1895, pp. 231-259. 
5 Atavus = grandfather of a grandfather; hence ancestor in the wider sense. 
