OF DIDELPHYS VIRGINIANA. 



137 



foot forward ; the palm and sole both resting on the ground. Were another deflected seg- 

 ment allowed, the fingers would point forward and the toes backward ; thus they are con- 

 sidered as reversing respectively the directions of the hand and foot. In this position, the 

 axis of each segment points in an opposite direction, both to the homologous segment and 

 its own succeeding segment, the salient angles at the flexures point in opposite directions, 

 as the elbow backward, the knee forward ; the front of the whole of one limb, as of its 

 sevei-al segments, corresponds to the back of the other, and vice versd : one is the reversed 

 repetition of the other. The radius and fibula, and the thumb and little toe, are all upon 

 the outside. We thus have, as homologous bones — 



ANTERIOR EXTKEMITT. 



POSTERIOR EXTREMITY. 



ANTERIOR EXTREMITY. 



POSTERIOR EXTREMITY. 



It cannot of course be denied that there are muscles whose general homologies cannot be 

 traced ; muscles existing in one species or group of mammals, no indication of which can 

 be found in other species or groups. But this fact does not detract from the pertinency of 

 homologies of other muscles that do exist, and are traceable. The biceps, for example, of 

 an opossum, is not the less the muscle so-called in a man or a monotreme, because an opos- 

 sum may have muscles that the other two want, or conversely. Muscles are so far under 

 the rule of teleology, that they may be, and are, frequently developed (according to what 

 by-play, so to speak, of morphology we are ignorant) in the face of special emergency, as 

 it were, to act definitely upon a particular organ or part, and are consequently not devel- 

 oped in those animals in which such particular organ or part is either wanting, or so far 

 modified as not to require such muscular apparatus. Similai-ly — narrowing our range of 

 observation, and turning from general to special homology, the expression of which we 

 would seek in different parts of the same animal as evidence of the further operation of 

 the law of antero-posterior symmetry that is found to hold with the bones — it cannot be 

 denied that we find muscles in the fore and hind limbs that thus far have resisted every 

 endeavor of ours to recognize then' correlation ; and the probability seems to be that there 

 really are, on either of the pairs of limbs, muscles that have no existence, actual or " poten- 

 tial," in the other — whether as the result of extreme teleological modification, or of mor- 

 phological difference, we do not presume to say. But such a fact should not be allowed to 

 militate unduly against evidence in favor of a law of fore and hind symmetry ; for non- 

 existence of this, or any other law, is not to be assumed because its application to every 

 detail cannot be shown. As far as I know, anatomists never have shown, and they proba- 

 bly never wUl show, the special homology of the highly differentiated and specialized mus- 

 cles of the cranial and caudal extremities of the trunk ; but this does not necessarily 

 preclude the belief that these parts are reversed repetitions of eacli other. And it may 

 fairly be assumed, I think, that there is an antero-posterior symmetry in the muscles of the 



MEMOIRS B09T. 800. NAT. HIST. VOL. II. 



35 



