THE GENETIC RELATIONSHIP OF PARASITES II 9 
and is furnished by a paleontologist. The case referred to is that of 
the parasitism of the gastropods on the crinoids and has been very 
clearly set forth by Clarke. Gastropods of the limpet type were 
parasitic on crinoids from the Silurian on to the latter part of the 
Paleozoic when all traces of such a condition ceased. The evidence 
points toward the cessation of this parasitic condition during Car- 
boniferous ages. Clarke believes "that all the positive and all the 
negative evidence we can now adduce on this deeply important sub- 
ject favors the presumption that the habit was abandoned or at least, 
to speak in terms of simple casuistry, was lost." There are no para- 
sitic snails on crinoids today, but close allies of the crinoids are para- 
sitized by gastropods of the limpet type. This, however, is held to be 
entirely independent of the former association between these groups. 
Much has been written concerning the nature and origination of 
parasitism but much remains for further experimentation before the 
factors involved can be understood. In order to form proper concep- 
tions it is necessary not only to consider the parasite and its weapons, 
but also the host with its defense, so that the question of the origin 
and nature of parasites is inseparably associated with those phenomena 
generally grouped under the term immunity. We must take into 
account also the effects of the environment upon both parasite and 
host. The available evidence indicates such a difference in the genetic 
physiology of parasites belonging to the higher and lower plants, re- 
spectively, that it may be best to discuss the groups separately. 
The parasitic seed plants form a series of isolated groups each of 
which bears a more or less evident genetic relationship to some inde- 
pendent group. Any theory of origination must take into account 
the undoubted existence of a number of disconnected points or levels 
oforigin. MacDougal has extensively investigated the conditions 
under which two plants may enter into the relation of host and para- 
site by growing one plant upon another and has concluded that "the 
ruling factor was in all cases the osmotic ratio between the sap of the 
two plants," a greater osmotic pressure being necessary in the incipient 
parasite. Of course many other factors are involved for one plant is 
not able to parasitize another simply because it possesses a higher 
osmotic pressure. It seems likely that parasites among the seed 
plants arose directly from independent ancestors without any inter- 
vening saprophytic condition such as probably existed in the evolution 
of the fungi. The first step intervening between independence and 
