GRE英语写作范文：
Evolution
ofsleep
　　Sleep
isvery
ancient.
Intheelectroencephalographic
sense
weshare
itwith
alltheprimates
andalmost
alltheother
mammals
andbirds:
itmayextend
back
asfarasthereptiles.
　　There
issome
evidence
that
thetwotypes
ofsleep,
dreaming
anddreamless,
depend
onthelife-style
oftheanimal,
andthat
predators
arestatistically
much
more
likely
todream
than
prey,
which
areinturn
much
more
likely
toexperience
dreamless
sleep.
Indream
sleep,
theanimal
ispowerfully
immobilized
andremarkab ...