viz.] PARASITISM, AN ACQUIRED HABIT 69 



The proof that parasites have acquired their 

 peculiar habit by actual adherence to other 

 plants is not easily derivable from actual experi- 

 ment, but it is based on much induction. I 

 have, therefore, gone somewhat into details of 

 anatomical structure to explain the nature of 

 parasitism, and to show the reader how im- 

 possible it is to conceive that it should arise 

 by " spontaneous congenital variation " without 

 any necessary reference to the host-plant itself, 

 as it must have done according to the Darwinian 

 theory. Nature, however, sometimes comes to 

 our help. M. E. Pe'e-Laby described 1 a remark- 

 able case in which a Passion flower (Passiflora 

 ccerulea, fig. 18), having its roots accidentally 

 in contact with those of a shrub of Euonymus 

 Japonicus, adhered to it ; a haustorial root 

 penetrated within that of the host and became 

 genuinely parasitic upon it, thus actually reveal- 

 ing how parasites can arise, and probably all 

 have originated in nature. Having thus estab- 

 lished the process, it has become hereditary 

 in all true parasites. 



The probable origin of many greenless Sapro- 

 phytes of temperate regions, is that they were 

 formerly parasites which have changed their 

 habit of life in now living solely on decomposing 

 organic matter, for we do not find that rich 

 humus or decaying vegetable matter has per se 



1 Revue General de Botanique, vol. xvi., p. 453. 



