THE GENETIC RELATIONSHIP OF PARASITES 
127 
changed but the sexual fusions have furnished the invigorating in- 
fluences which are responsible for pleomorphy and the pleophagic 
habit. It cannot be denied that sexual fusions should furnish in- 
vigoration but those fusions found here which appear to be a degenerate 
substitution for a former fertilization do not seem adequate to account 
for such a profound change in the life of a parasite. 
If we are to take the view that the present aecial host is the original 
and primary one, then it would seem reasonable to expect the aecial 
stages to be on the more primitive hosts as a rule. The condition 
found in our North American heteroecious forms does not bear out 
this expectation for we find out of ninety-nine species that seventy-eight 
have aecial hosts standing higher in classification than their telial 
hosts. To support the argument that the gametophytic host was the 
original autoecious one, it has been urged that there are instances 
where short-cycle forms are known on the aecial hosts of heteroecious 
species, the teliospores of the two showing structural parallelisms so 
striking that some kind of relationship is strongly suggested. Puccinia 
mesnieriana a short-cycle form occurring on Rhamnus and having 
teliospores peculiarly like Puccinia coronata, a heteroecious grass form 
having its gametophytic stage on Rhamnus, has been cited. There are 
numerous other cases. Puccinia Xanthii is an autoecious form on 
Xanthium, which harbors the aecial stage of a sedge rust, Puccinia 
Cyperi. Polyth3lis fusca, a, short-cycle form on Anemon2 ,h.a.ste\iospores 
identical with Tranzschelia punctata which has its telia onPrunussind its 
aecia on Anemone. Numerous other cases could be mentioned. May 
the evident genetic relationship best be explained by amplification or 
reduction? Not only does the latter more nearly conform to the 
physiology of parasitism but also agrees with certain conceptions of 
differentiation which have been formed from experimental genetics. 
Based upon niany considerations we find Bateson saying that he feels 
no reasonable doubt that we may have to forego a claim to variations 
by addition of factors, yet variation by loss and fractionation of factors 
is a genuine phenomenon of contemporary nature. If there is a possi- 
bility that we may have to dispense with additions from without in 
independent forms how much more strongly does such a situation 
press its claim for recognition in parasites. To return briefly to the 
associated long- and short-cycle forms above mentioned we find that 
no startling variation has to be assumed in order to derive the latter 
from the former. If on the gametophytic host, following the sexual 
