360 Professor H. H, Swinnerton— | 
provide the framework both for verbal description and for graphic 
representation. 
In any series of changes thus elucidated the range is limited in 
extent and hes between two extremes, which may be usually 
ascertained for any group of organisms. Thus the number of plates 
in a transverse row of ambulacrum is not known to be less than two 
or more than twenty. The extreme range of shape, moreover, does 
not normally extend beyond the limits depicted in Fig. 1, I and III. 
There may be special groups, such as irregular echinoids, in which the 
modification of these plates may be carried to greater extremes. 
But even among them the range is limited. This is true also for the 
structural units in any group of organisms, as well for the number of 
toes or of teeth or the pattern of the dental crown in mammals, 
as for the degree of coiling or complexity of suture line in ammonites. 
Tn all the great orders, as in the Echinoids, there are special groups 
in which the range for any one structure may be excessive, as, for 
example, the teeth of the Proboscidea, and the suture line of the 
Pinacoceratidee. But even in these cases the range will be found to 
have its limits. 
Further, it must be remembered that, within the bounds of any 
group of allied forms, the number of directions of change for any 
particular structural unit is likewise limited. Numerical characters 
can only increase or decrease in number, coiling of a shell can only 
be in a plane or in a cork-screw spiral up to a certain height. | This 
is merely putting the case for orthogenesis and rectigradation from 
the standpoint cf individual structures, rather than from the stand- 
point of the organism as a whole. What is a variation in an organism 
is often merely another step in the serial change of a structural unit. 
Hitherto the term structural unit has been used as though it 
always stood for something that was quite clear and precise. This, 
however, is not the case. Serial change may take place on a basis 
that is too vague and ill-defined for description as a structural 
unit. The ornamentation of an ammonite shell exhibits quite well- 
defined series of changes—but what is the structural unit which 
changes ?- Examples of this kind serve to emphasize the importance 
of thinking less statically and more dynamically, less in terms of 
structures that are present or absent, and more in terms of characters 
that are changing. 
MorPHOLOGICAL GRAPHS AND THEIR INTERPRETATION. 
In order to explain the use of the principles just enunciated for 
the purposes of graphical representation, the unit series of changes 
recognizable in any type of organism may be represented by the 
capital letters, A, B, C, ete. Thus, in the illustration already taken 
from Paleozoic echinoids, the series of changes shown in Fig. 1, 
I-III, may be called A. The series showing increase in numbers of 
plates in a transverse row may be called B, and so on. 
The steps in each series may be taken as they stand, or they may 
