In the face of such evidence of rapid evolution and diversification of
morphological traits,can we assume that behavioral traits could remain
stable over very long periods of time? If we seek common mechanisms
for behavior in humans and birds,can we assume that they survived
despite the 300-million-year interval available for their divergent evolu-
tion? Such an assumption turns out to be at least not unreasonable,
because the nervous system is one of the more conservative biological
systems with respect to evolutionary change.Furthermore,the pheno-
typic condition is determined by significant environmental editing of
the genotypic blueprint that governs the developing nervous system.
Homologies in the control of behavior can exist even in the face of very
extensive diversity of form and function of the whole organism.Let me
explain with examples from the function of sensory systems.
Sensory cells of each type—retinal rods and cones in the case of vision
(Polyak 1957) and cochlear hair cells,which are mechanoreceptors,in
hearing (Stebbins 1983)—evolved from common roots in all mammals.
Genetic blueprints,as it were,presumably set the main distinct features
of each of these classes of cells,including the number to be produced in
an individual animal.However,important differences exist among
species.
These are quantifiable morphological traits,and it may help to look at
some numbers.Surprising uniformity is seen among mammals in the size
and number of hair cells in the cochlea of each ear,always a tiny organ
usually with two or three turns in the spiral cochlea,about 35,000 hair
cells,and the same number of bipolar neurons in the spiral ganglion,
which lies beneath the cochlea.The visual system varies in the size of the
eyeball,retina,and numbers of rods and cones.The vertebrate eye as a
camera is an example of a great uniformity,but eyes are more variable
in detailed structure in different species,reflecting the place of vision
in the lives of the species.The number of rods relative to the number of
cones varies enormously,with some nocturnal species having eyes con-
sisting almost entirely of rods,and diurnal reptiles and birds having eyes
consisting almost entirely of cones.The human eye is a fairly typical
anthropoid primate eye,and its numbers,which are the same in rhesus
monkeys,are impressive.I have seen no counts of the number of nerve
cells in the neural retina,but it certainly numbers in the millions;about
one million ganglion cells;that is,cell bodies of neurons make up the one
million fibers in each optic nerve (cranial nerve II).One guesses that the
retina has between five and ten million additional nerve cells.Each eye
has about 100 million rods and about 7 million cones.To emphasize
diversity,I should add that primate eyes are atypical among mammals in
having many more cones and a fovea centralis for improved detail vision
in the center of the visual field.The singularity of anthropoid primates181 Paleoneurology and the Biology of Music