As will be indicated later on, spines may also arise through
the operations of a number of forces and conditions, and it
may well be asked, therefore, Do spines have any profound
significance? It must be granted at the outset that apart
from other characteristics, or when regarded as simple spini-
form extensions of certain tissues or organs, they have no
such value or meaning. How, then, should they be con-
sidered? ‘The reply is evident: Their importance lies not in
what they are, but in what they represent. They are simply
prickles, thorns, spines, or horns; they represent, as will be
shown, a stage of evolution, a degree of differentiation in the
organism, a ratio of its adaptability to the environment, a
result of selective forces, and a measure of vital power.
After studying numerous organisms, the writer is led to
believe that in every case no single reason is sufficient to
account for this spinose condition. The original cause may
not be operative through the entire subsequent phylogeny,
so that spines arising from external stimuli and then serving
important defensive purposes may at a later period practi-
cally lose this function; or spines may become more and
more developed simply by increasing diversity of growth
forces, or through the multiplicity of effects. In this way
causes may follow, overlap, or even coincide with each other;
but in interpreting special cases the problems involved may
be quite complicated and often obscure.
