260 BOTANICAL GAZETTE [OCTOBER 
crinoid of earlier times and the gastropods and sea urchins of the 
present. It is made clear by CLARKE that the limpets of the early 
Silurian were largely parasitic on the crinoids, a habit that per- 
sisted for millions of years, until the closing stages of the Paleozoic, 
when evidences of it were lost, and no traces of parasitism of 
snails on the few crinoids of the present are known. Other gas- 
tropods of the limpet structure are now parasitic on the starfish 
and sea urchins, close relatives of the crinoids. The earlier lim- 
pets were not carried beyond the stage of possible regression in 
their parasitism, but the modern parasitic gastropods are ‘often 
so modified by their degeneracy that their nature is hardly recog- 
nizable, and this parasitism is fixed and beyond repair.” Two 
separate cases of adaptational adjustments seem involved, and 
the parasitism of the modern gastropods is taken to be wholly 
independent of the earlier assumption and abandonment of the 
habit. The suggestion lies near, however, that a family which 
has thus furnished two separate series of parasites is one which 
by morphological characters or physiological tendencies is liable 
to assume dependent relations with other organisms as hosts. 
Parasitism among the higher plants of the present time is con- 
fined to ten families, one of which has been added recently to the 
list by the work of Dr. W. A. CaNnNon. It may be safely assumed 
that in some of these, the Orobanchaceae, for example, the habit 
is far beyond retraction. 
Desert LABORATORY 
Tucson, ARIZONA 
