THE GENETIC RELATIONSHIP OF PARASITES 
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pre ted finds it necessary to qualify the term. Cook and Swingle have 
pointed out that ''normal and long-sustained evolutionary progress is 
not accomplished on single or narrow lines of descent, but is possible 
only for large companies of interbreeding individuals; that is to say, 
for species." Certainly in all organisms where we have interbreeding 
and interweaving of distinct lines of descent there is such a thing as a 
real specific group but in the parasitic fungi all traces of the coherence 
of such a group has been lost. With the exception of the mucors, 
where the interweaving of strains is definitely known, and the possible 
occurrence of sexual strains in some downy mildews and anthracnoses, 
there is no suggestion of groups of interbreeding individuals. The 
individuals continue their existence without relation to other individ- 
uals. It is clear that man makes the species here solely for his con- 
venience and he must decide whether he will make morphology or 
physiology his standard. If it is true that progress cannot be main- 
tained along narrow lines of descent this will assist in understanding 
why advances are not to be found in the parasitic fungi for here the 
lines are the narrowest possible, simply the continuations of individual 
existences. If we are given an individual spore we can start a life-cycle 
which will bring us back to more spores of the same sort and we may 
raise generation after generation but they are descendants of a pure 
line in no way influenced by individuals of different origins. The 
factors which may influence them are their hosts and the environment. 
Through morphology we form our judgment concerning the possible 
similar origin of individuals which come to us to classify. In the 
parasitic fungi as a rule it is only the reproductive features that receive 
morphological consideration. If we considered only dead specimens 
there would be no difliculty,but with the advent of experimental culture 
work, perplexing questions have arisen. The work of Eriksson, 
Klebahn, and others served to emphasize this matter but it is clear 
from DeBary's writings that he was aware of the specialization of a 
parasite to a host although he did not use the form of expression now 
prevalent. Morphological equivalents growing upon the closely allied 
hosts may not have the same behavior. The powdery mildews, an- 
thracnoses and rusts furnish many familiar instances. This means that 
the plant body is coming in for attention and that the consideration 
must be of a physiological nature. The question whether forms struc- 
turally alike but reacting physiologically differently should be regarded 
as belonging to the same species or a race, etc., may be an interesting 
