CHAPTER XLIV. 
THE VASCULAR SKELETON. 
PassinG from the characters of external form to the internal arrangement 
of tissues, the Vascular system provides by far the most constant 
structural characters; and, as it is naturally the best preserved tissue in 
the fossils, it gives a basis for comparison of both ancient and modern 
Pteridophytes. But in dealing with anatomical facts it must be 
remembered always that in any progressive evolution vascular structure 
follows, and does not dictate external form: all the evidence which it 
yields is necessarily ex post facto evidence. On the other hand, the 
structural effect of a certain development may persist even after the 
formal characters with which it was primarily bound up may have been 
altered or even wholly removed. In fact, anatomical characters are apt 
tardily to follow evolutionary progress, and to thereafter persist; they 
possess what may be described as a sort of phyletic inertia. 
It has already been shown in Chapter XV. that the prevalence of a 
central stele in the axis of Vascular Plants is in direct accord with a 
strobiloid theory of the primitive shoot: and that the strictly cauline origin 
of the central region of the stele, and the insertion of the leaf-traces upon it 
with but slight disturbance, as seen especially in the smaller-leaved forms, 
are also features which harmonise with a strobiloid theory: the facts were 
held to suggest a primitive condition in which the axis was the dominant 
factor, and the appendages of subordinate importance. This position 
receives additional support from the demonstration given above in 
Chapter XLII., that the axis is the first part to be structurally defined 
in the initiation of the embryo. But it will be necessary further to 
show how far the Vascular structure of the larger-leaved types will accord 
with a strobiloid origin. The leading anatomical facts required for this 
are contained in the special descriptions of the several groups in Part II. ; 
they may now be drawn together into a short collective statement. 
By a general consensus of opinion, the non-medullated monostele is 
recognised as the primitive stelar type, and it has been shown severally 
