There  can  be  little  doubt  that,  in  the  long  run,  such
individuals  will  survive  as  are  best  fitted  to  cope  with  the


xin  NATURAL   SELECTION  227

conditions  to  which  they  are  subject.  According  to
Darwin's  theory  of  natural  selection,  it  is  assumed  that
such  surviving  individuals  would  transmit  their  special
properties  to  their  progeny,  and  there  would  thus  be  a
gradual  approximation  towards  a  perfect  adaptation  of  the
species  to  its  surrounding  conditions  by  virtue  of  this
"  survival  of  the  fittest."

Let  us  suppose  the  conditions  to  change.  Gradual  alter-
ations in  climate  and  other  conditions  are  known  to  take
place,  owing  to  subsidence  or  elevation  of  the  land.  But
conditions  might  be  changed  in  many  other  ways  :  some
animal  or  plant  previously  used  as  food  might  become  ex-
terminated, or  a  new  enemy  might  find  its  way  into  the
district  inhabited  by  the  species.  Then  such  individuals
as  presented  variations  which  enabled  them  better  to  cope
with  the  new  surroundings  would  have  the  advantage  over  the
others,  and  would  have  a  much  better  chance  of  surviving
and  leaving  progeny.  The  useful  variations  thus  produced
and  transmitted  to  the  progeny  would  tend  to  increase,
generation  after  generation,  until  a  form  sufficiently  distinct
to  be  regarded  as  a  new  species  had  become  developed  from
the  original  one.
