THE GENETIC RELATIONSHIP OF PARASITES^ 
Frank Dunn Kern 
Parasitism is a mode of life having many forms and degrees. Nu- 
tritional dependence may be partial, temporary, or complete. All 
possible intergradations exist. It has been estimated that a large 
percentage of the total number of seed plants may use complex food 
derived from other organisms and yet only a comparatively few as- 
sume a mode of life which has been regarded as parasitic. Gradations 
also connect the organism, which is dependent upon other living or- 
ganisms for nourishment, with those which are dependent upon or- 
ganic, but non-living, food. 
Organisms may be classified physiologically as parasites, sapro- 
phytes, and autophytes (or holophytes) but such a classification does 
not coincide with a taxonomic one. Upon morphological grounds 
parasites are variously distributed in the taxonomic series with inde- 
pendent or saprophytic forms. If our taxonomic classifications can 
be regarded as representing phylogenetic relationships their arrange- 
ments not only suggest the evolution of parasites from free-living an- 
cestors but also indicate many separate points of origin. The presence 
of free stages in the life-cycle of many parasites; the frequent occur- 
rence of temporary parasitism; and the close morphological resem- 
blances between some independent and some dependent forms afford 
abundant evidence to justify such a view. 
Organic dependence may have begun gradually or it is possible in 
some instances that the time element may not have been a factor and 
that parasitism may have come about as a sudden mutation. Plaus- 
ible explanations of the origin of the parasitic habit in animals do not 
seem so difficult as in plants. There is the possibility that the seeking 
of shelter may have been the starting point for some animal forms, 
others may have found protection accidentally having been swallowed 
in an embryonic or even adult condition. It has been suggested that 
^Invitation paper read at the symposium on "The Genetic Relationship of 
Organisms" before the Botanical Society of America at Philadelphia, December 30> 
19 14. Contributions from the Department of Botany, Pennsylvania State College, 
No. 3. 
116 
