Igtt] M ACDOUGAL—PARASITISM 259 
position where the only obvious way to extinction would be by 
the destruction of their hosts, resulting from their own effective- 
ness. Nothing known of the life history of any of these forms 
suggests a possible aban- [ — 
donment of the parasitic | 
habit and of an advancing 
morphological develop- 
ment. So far as the higher 
plants are concerned, the 
only consideration hitherto 
given to parasitic forms has 
been to view them as pass- 
ing down an inclined plane 
of atrophies, which would 
ultimately lead to their ex- 
tinction, without reference 
to the abundance of devel- 
opment of the host forms. 
No hint has yet been ob- 
tained as to the possibility 
of a retracement, by which 
a dependent might once 
more regain its standing as 
an autophyte. 
Regressive action of this 
character would naturally 
be discernible only in a 
Series of material extend- 
ing over long periods of 
time, such as that obtained 
by the paleontologist. It 
is interesting to note that 
this subject is one to which 
some serious attention has 
been given, and Dr. J. M. CrarKe* has recently summarized 
the information with regard to the case of the limpet and the 
—Cissus laciniata parasitic on 
Fic. 
Opuntia Blakeana (see fig. 2). 
a 
4 The significance of certain early parasitic conditions. Science 33:291. TQTT. 
