129.  Classification  Based  on  Resemblances.  —  It  is  evi-
dent from  the  foregoing  illustrations  that  the  classification
of  animals  and  plants  is  based  on  the  fact  that  there  are
certain  resemblances  in  structiu'e.  Thus  all  vertebrates  are
alike  in  having  a  backbone  and  some  other  structures  not
found  in  lower  animals ;  all  mammals  are  alike  in  having  not


CLASSIFICATION  OF  ANIMALS  AND  PLANTS     139

only  a  backbone,  but  also  hair,  milk-organs,  and  diaphragm ;
all  carnivores  are  alike  in  having  mouth  and  digestive  organs
adapted  to  meat-eating  in  a  way  not  found  in  other  mammals ;
and  dogs  and  wolves  are  nearer  alike  in  all  respects  than
they  are  like  any  other  animals.     It  is  all  a  question  of  re-
semblances— resemblances  in  detail  between  individuals  of
a  si>ecies,  and  more  and  more  general  resemblances  as  we  pass
from  species  to  larger  and  larger  groups  (genera,  orders,
classes),  imtil  we  find  in  divisions  or  phyla  only  such  general
resemblances  as  the  backbone,  which  characterizes  vertebrates.
These  resemblances,  or  points  of  structure  which  mark  the
animals  or  plants  belonging  to  any  particular  group,  are  the
characteristics.    For  example,  the  backbone  is  a  characteristic
of  vertebrates,  for  no  lower  animals  have  it ;  and  hair,  milk-
glands,  and  the  diaphragm  are  characteristics  of  mammals;
the  formation  of  seeds  in  a  peculiar  manner  is  a  characteristic
of  seed-plants ;  the  arrangement  of  flowers  in  a  peculiar  head
is  a  characteristic  of  composites.     So  for  each  large  or  small
group  of  animals  and  plants,  from  species  to  primary  divisions,
there  are  distinguishing  marks  based  on  resemblances.
