250  Hybridization  in  Plants.  j AlApriir'i92iarm' 
that  illustrate  individual  variation  in  a  species  should  be  kept  in 
mind. 
Regarding  the  possibility  of  crossing  between  distinct  varieties, 
species  or  genera,  it  might  be  said  that  hybridization  seems  in  many 
cases  easy  between  varieties,  sometimes  easy  and  at  other  times  diffi- 
cult between  species  and  only  rarely  possible  between  genera.  When 
the  last  is  accomplished,  the  progeny  is  spoken  of  as  a  bi-generic 
hybrid. 
Apparent  obstacles,  however,  to  crossing  even  varieties  seem 
at  times  to  exist.  Thus,  colors  of  diverse  kinds  may  form  a 
strong  barrier.  As  first  pointed  out  by  Charles  Darwin,  the  scarlet 
pimpernel  (Anagallis  arvensis)  may  grow  in  abundance  with  the 
blue  pimpernel  (Anagallis  caerulea),  the  former  with  bright  scarlet, 
the  latter  with  azure  blue  flowers,  but  both  in  the  wild  state  and 
under  cultivation  hybridization  seems  difficult.  But  that  crossing 
may  be  effected  is  shown  by  the  ugly,  lurid  garden  progeny,  Ana- 
gallis Phoenicia. 
Similarly,  a  cross  between  the  scarlet  cardinal  flower  (Lobelia 
cardinalis)  and  the  blue  Lobelia  siphilitica  is  extremely  rare  in  the 
wild  state,  and,  when  secured  under  cultivation,  the  flowers  of  the 
hybrid  possess  a  lurid,  ugly,  pinkish  purple  color.  In  this  instance 
the  intermixture  of  deep  orange  pigment  in  the  cell  chromoplastids 
with  dissolved  alkaline  blue  pigment  is  a  semi-incompatible  condi- 
tion. 
Again,  hybridizers  have  often  observed  that  some  families 
contain  species  that  cross  readily  while  other  families  seldom  hybri- 
dize. Of  the  former  it  seems  generally  the  case,  that  when  the 
family  includes  regular  and  irregular  flowered  genera,  the  former 
cross  with  difficulty,  the  latter  with  ease.  For  example,  in  the 
Geranium  family,  Geranium  and  Erodium  fail  to  cross  in  their 
species,  while  Pelargonium  crosses  very  readily. 
Again,  Rhododendron  and  Azalea  species  with  irregular  flowers 
cross  readily,  while  crosses  between  regular  flowered  members  of 
the  Heath  Family  are  rare. 
Crosses  between  even  diverse  looking  species  of  a  genus  may 
often  be  effected  with  ease  and  the  hybrid  progeny  often  blends  the 
characters  of  both  parents  in  even  numbers.  Thus  V erbascum, 
Dianthns,  Pelargonium,  Sarracenia,  Nepenthes  and  many  others 
blend  readily  even  between  the  most  diverse  species  and  the  progeny 
is  largely  fertile. 
Crosses  between  genera  have  only  rarely  been  effected  and 
