1895. ] NEW YORK ACADEMY OF SCIENCES. 239 
in case of any given human muscular variation, a muscle of 
similar character is found in one of the lower vertebrates does 
not warrant the assumption that both are derived by inheritance 
from an immediately precedent common ancestral form. The 
form in which the variant human muscle appears normally may 
be incalculably far removed from man, may even belong to a dif- 
ferent vertebrate class. That the structural coincidence of the 
two muscles is to be taken as indicating anything more than the 
most generalized relationship of vertebrates is difficult to be- 
lieve. For many of the aberrant muscular conditions observed 
in man a very comprehensive view as to their derivation must be 
adopted. I believe that we are right in referring such varia- 
tions, as will be considered in detail below, to the development 
of an inherent constructive type, abnormal for the species in ques- 
tion, but revealing its morphological significance and value by 
appearing as the normal condition in other vertebrates. 
The question, as far as it affects the variations to be considered, 
may be represented graphically somewhat in the following man- 
ner: 
a b a b 
© y © y 
Fic. 1. Type form of Muscle. Fic. 2. Cleavage variation. 
In Fig. 1 let the line a—} represent the skeletal origin, the 
line x-y the corresponding insertion of a muscular plane. 
Considering this arrangement as the type, in which the entire 
space between origin and insertion is occupied by an uninter- 
rupted muscular plane, it will become apparent that modifica- 
tions of this type can take place in two ways. 
1. Cleavage variations, retaining in general the original scope 
of origin and insertion, in which the original muscular plane ap- 
pears as two or more distinct muscles. (Fig. 2.) 
2. Reduction variations, where a portion of the origin or of 
