THE GENETIC RELATIONSHIP OF PARASITES 
123 
produce spores usually unlike one another and are all for purposes 
of propagation. Pycnia produce simple spores unlike any of the 
others and of unknown function. Autoecious forms may exhibit the 
same number of spore-forms and in the same sequence as just de- 
scribed for heteroecious forms, or they may have the aecia and ure- 
dinia lacking. In this case the pycnia, when present, are followed 
at once by telia and we have what may well be designated as a short 
life-cycle. The heteroecious forms always show well developed 
pleomorphy whereas the autoecious forms may or may not. The 
presence or absence of urediniospores, which simply repeat themselves 
over and over, is held to be of no fundamental importance in the forma- 
tion of a life-cycle. It is understood that telia are always present and 
that pycnia usually (but not always) follow from telial infection. If a 
heteromorphic spore-stage (aecia) follows the pycnia the form is 
thrown into the long-cycle series without regard to the presence or 
absence of a second heteromorphic stage (uredinia). 
Ever since heteroecism has been definitely known to exist in this 
group, or for fifty years, naturally there have been speculations 
concerning its origination. Within the last ten years since sexuality 
and an alternation of generations have been demonstrated the char- 
acter of the speculations has been somewhat modified. The probable 
evolutionary tendencies within such a remarkable group of obligate 
parasites must have an important bearing upon our general conception 
of the genetic physiology of parasitism. 
If we accept a view, which has found much favor, that the complex 
heteroecious types have been derived by amplification from the simple, 
autoecious, short-cycle forms, we then find it necessary to explain how 
parasites, in which degeneration might be expected, have made un- 
precedented advances. To be sure it seems most logical to derive the 
complex from the simple and the amplification, since it is in the sporo- 
phytic stage, is in accord with the conditions found in independent 
plants. But the questions immediately arise whether we can expect 
such obligate parasites to parallel the development of higher plants 
and whether we need to make such assumptions in order to explain 
present conditions. In the light of our observations on the behavior 
of parasites it seems that we ask a good deal of both evidence and 
probability in seeking to align these parasites with the independent 
higher plants on the basis of the development of the alternating 
generations. 
