is an aspect of their almost specific adaptation among mammals for
diurnicity.
Sensory cells are homologous certainly for mammals and perhaps
for all vertebrates.I refer specifically to rods and cones in vision and
mechanoreceptors that are auditory hair cells.These sense cells function
in essentially the same way at some levels of organization of behavior,
whereas other levels reflect the diversification that has occurred.Diver-
sity lies in the way that neural connections made by these cells are orga-
nized into systems involved in controlling vision and hearing.Yet the
pathways between sensory cells in the retina and nerve cells in the retina
and brain are fundamentally the same in all mammals until one reaches
perhaps third- or fourth-order neurons in the synaptic pathway,when it
reaches mammalian cerebral cortex.(Recall that cortex does not exist as
a distinct structure in other vertebrates;neurons homologous to these in
nonmammals can be identified as reaching forebrain structures such as
the Wulstin birds [Butler and Hodos 1996]).I present these few details
to suggest the complexity of the underlying system and do not pretend
that it is a complete description.
The “instructions”that determine the pattern of connections at all
levels are partly genetic and partly environmental.Genetic instructions
set the number of nerve cells and “tell them”to grow by arborization,
by sending out fibrils like branches of trees.Genetic instructions may
also specify some environmental features,such as the nature of pathways
along which cell growth can occur.Environmental instructions are in the
actual formation of pathways,end points at which cells can make synap-
tic connections with other cells,and the amount of use to which cells
are put,or extent to which they are stimulated by environmental events.
Unused synapses may disappear,and unstimulated nerve cells may
simply die.Cell death is an important phenomenon in the construction
of the mature nervous system,a kind of editing of unnecessary connec-
tions.This is the kind of nature-nurture interaction that one may assume
to be the basis of the evolution of musical expression and of some aspects
of musical experience.Let us now consider some aspects of the organi-
zation of living brains in which that evolution occurred,the extent to
which brains diversified,and the extent to which we can recognize uni-
formities in their organization.Brain Organization:Uniformities
How much diversification of neurobehavioral traits can be expected to
occur over the many millions of years available for their separate evo-
lution? This is a general evolutionary issue for which no simple answer182 Harry Jerison