482 
Now,  as  the  malarial  parasite  is  unable  to  leave  the  human  body 
by  the  skin,  the  lungs,  the  bowels  and  the  kidneys,  it  is  impossible 
for  it  to  be  communicated  directly  from  one  individual  to  another, 
and  it  would  eventually  tend  to  die  out,  as  has  actually  happened 
in  many  parts  of  the  world. 
But  Nature,  which  invariably  provides  for  the  propagation  of  the 
species  under  suitable  conditions,  has  here  also  made  provision  for 
the  malarial  parasite,  and,  as  shown  by  Professor  Ross,  a  special 
species  of  mosquito,  the  Anopheles,  steps  in  as  an  intermediary. 
If  we  again  examine  the  blood,  we  find  that  certain  of  the  malarial 
parasites  do  not  follow  the  cycle  which  I  have  described  and  sporulate, 
but  form  crescentic  or  rounded  bodies  carrying  brown  pigment,  which 
are  easily  distinguished  under  the  microscope.  These  are  the  Sexual 
forms.  When  an  Anopheline  mosquito  feeds  on  an  individual  having 
these  particular  forms  in  his  blood,  it  draws  a  number  of  them  into 
its  stomach,  where  they  undergo  certain  changes,  and  under  favour¬ 
able  circumstances,  in  about  a  week,  form  threadlike  bodies  or  spores 
which  eventually  find  their  way  to  the  salivary  glands  of  the  mosquito 
situated  at  the  base  of  the  proboscis  or  trunk. 
The  mosquito  has  thus  become  ‘  infected,’  and  when  it  next  bites 
an  individual,  at  the  moment  when  it  inserts  its  proboscis,  it  injects 
some  of  the  salivary  fluid  loaded  with  spores,  which  thus  obtain 
admission  To  the  blood,  attack  the  red  cells,  and  the  cycle  in  the 
human  body  which  I  have  already  described  begins  once  more,  and 
the  individual  suffers  from  an  attack  of  Malarial  Fever.  And  thus 
the  vicious  circle  goes  on — man  to  mosquito,  mosquito  to  man. 
It  is  clear  then  that  for  the  continuance  and  spread  of  Malaria 
two  factors  are  required,  and  it  is  essential  to  recognise  the  fact  that 
there  are  two,  namely-,  (i)  an  individual  whose  blood  contains  the 
sexual  forms  of  malaria,  i.e.,  an  infected  individual ;  and  (2)  a 
species  of  mosquito,  the  Anopheline,  which  in  its  turn  must  also 
become  infected,  i.e.  an  infected  mosquito.  Do  away  with  one  or 
other  or  both  of  these  factors  and  Malaria  instantly  disappears. 
If  we  have  no  infected  individuals,  it  does  not  matter  how  many 
Anophelines  there  are  in  a  locality,  they  cannot  become  infected,  and 
consequently  cannot  carry  malaria ;  and  conversely,  if  there  are  no 
Anophelines,  it  is  immaterial  how  many  infected  individuals  may  be 
introduced  into  a  locality,  the  disease  cannot  be  transmitted  and  must 
inevitably  die  out. 
