MUSCULAR SYSTEM 121 
In general, the principle holds good that those muscles 
are most subject to variation which can be dispensed with 
without disturbance or disadvantage to the organism as a whole, 
either because they can be easily replaced by other muscles, 
or because they have only a subordinate part to play. In 
illustration of this I would merely refer to the pyramidalis, 
the abortive caudal muscles, the muscles of the pinna, the 
palmaris and the plantaris, the vestigial character of which 
clearly points to their ultimate complete suppression. 
Research has shown, however, that it is not only to the 
retrogressive tendency of the muscles that variation is due, but 
that variation may in some cases indicate a tendency to 
progressive development. The best example of this is afforded 
by certain flexor muscles, and by the flexor longus pollicis, and 
the gluteus magnus. 
A third kind of variation occurs, in those cases in which a 
tendon may return to former points of insertion on neighbouring 
bones, e.g. the rectus abdominis is occasionally inserted on to the 
more anterior ribs. And to the same category belong the 
splitting off of the abductor hallucis from the tibialis anticus, 
which occurs in very varying degrees. 
All these cases, which must be denominated reversionary, 
indicate the extraordinary tenacity with which certain 
peculiarities persist and are repeatedly passed on from one 
generation to another. This power of reproduction must, however, 
necessarily grow weaker, as an organ in course of time loses its 
original functions in adaptation to new ones. As a consequence 
of this, attempts at reconstruction necessarily become more and 
more imperfect. 
The same is the case with many other muscles (eg. the 
sternalis, levator claviculze, latissimo-condyloideus, and epitrochleo- 
anconzeus) which now only rarely occur in Man, and which, when 
they are present, furnish important indications of a long-past 
period in the development of the human race. 
There is no good ground for doubting the possibility of the 
hereditary transmission of muscular anomalies, although, as Testut 
rightly remarks, the difficulty of obtaining material for a direct 
proof is evident. The difficulty in this case is greater than in 
that of mere external variation, such as pigmentation, different 
coloration of the opposite eyes, abnormal hairiness, birth-marking, 
polydactyly, and others akin to these. 
It is reserved for future investigators to add to our as yet 
