358 Professor H. H. Swinnerton— 
suggestions for such a method it will be necessary, first of all, to give 
some account of the principles which underlie a suitable method. 
This will be followed by several examples illustrating the application 
of the method. Finally, sundry thoughts suggested by these examples 
will be discussed. 
StRucTURAL ELEMENTS AND FLow LINES oF CHANGE. 
No one can study the anatomy and development of several types 
of allied organisms, living or extinct, without realizing that they 
consist of various parts, each of which, whilst it is correlated with the 
others, undergoes changes and modifications that are peculiarly 
its own. Such parts may be referred to as structural elements or 
units. In a broad sense they may be anything from a single cell 
to a complete organism, from a cell that becomes a muscle fibre, to 
a tadpole that becomes a frog. In the narrower sense in which the 
term is used in this paper they may include any relatively simple 
part, as a digit; or relatively complex part, as the whole limb of a 
vertebrate ; a single segment, or the whole glabella or pygidium of 
a trilobite. 
The term organ is not synonymous with structural unit. An organ 
such as the eye, or fin, may be treated as one structural unit, or as 
consisting of several units, according to the object of the particular 
Inquiry. 
When a structural unit is followed through the life-history of an 
organism, it is seen to undergo a series of changes, which is usually 
described as its course of development. If, on the other hand, it 
be examined in a number of more or less closely allied genera and 
species, which are either contemporaneous or follow one another 
in time, it is found to exhibit a series of conditions often strikingly 
like those which are seen at different stages in development. 
By such developmental and comparative studies it is possible to 
reconstruct the series of changes to which any given unit is subject. 
The problem is analogous to that of the stratigraphical geologist 
seeking to work out the sequence of the rocks. A complete and 
unbroken sequence in one exposure 1s as rare as a similarly perfect 
lineage in a museum. He must therefore attain his end by piecing 
together the information obtained from several exposures. So, 
likewise, the paleontologist may discover the sequence of changes 
experienced by individual structural units by piecing together 
fragments of information obtained by the comparison of allied forms, 
and from portions of imperfectly known lines of descent. An 
illustration will perhaps make this clearer. 
A comparison of a large number of paleozoic echinoids ' reveals 
the fact that the ambulacral plates assume various shapes. When 
these are placed side by side they can be arranged to form a series 
of which several steps are shown in Fig. 1, I-III. A further series 
can be recognized which follows the lines of an increase in the number 
1 R. T. Jackson, Mem. Bost. Soc. Nat. Hist., 1912. 
