THE GENETIC RELATIONSHIP OF PARASITES 
117 
in many cases the habit may have begun ''with the naturally more 
sluggish females, prompted not by hunger, but by the impulse to seek 
some conveniently sheltered place for the birth of the young." It 
may be possible eventually to gain some usable hints from the past 
ages but parasitic conditions are so dependent upon adaptations in the 
physiology and soft structures that the record in the rocks is exceeding- 
ly fragmentary and difficult to interpret. 
When parasitism is suggested the one feature concerning the para- 
site which presents itself most strongly is that of degeneration. The 
dogma of degeneration is so thoroughly drilled into the minds of the 
students of biology everywhere that parasitism and degeneration seem 
unalterably linked together. We are prepared for the expressions 
that parasites *'are more or less degenerate according to the extent of 
their parasitism" or that parasitism "unfailingly involves degener- 
ation." Although we may expect old and popular beliefs to be over- 
thrown at any time there seems to be much in favor of the correctness 
of this one. So far as the seed plants are concerned there is evidence 
of somatic degeneration, consisting chiefly of reductions or atrophies 
of the shoot and root, even in those forms which are only partially 
parasitic; while those which are fixed parasites may be still further re- 
duced, the root failing to develop, the shoot remaining unbranched, 
the leaves lacking chlorophyll and frequently appearing as colorless 
bracts. With regard to the spore plants it is not so easy to form judg- 
ments concerning possible somatic modifications since there are likely 
to be long genetic series of heterophytic forms and no clearly related 
autophytic groups to form a basis for comparison. 
As regards sexuality it has been a widely accepted doctrine that 
the parasitic mode of life leads to its disappearance. Investigations 
by Ernst and Schmidt on the root parasite Rafflesia have caused them 
to draw the general theoretic conclusion that so far as seed plants are 
concerned parasitism does not tend to the disappearance of sexuality. 
Other studies have, however, shown that the fruits and seeds may 
show various specializations. In the case of the fungi we are not sur- 
prised to find in an authentic text-book, now only fifteen years old, 
the statement that sexual reproduction is common in the Phycomy- 
cetes, occasionally occurs in the Ascomycetes, and is not certainly 
known to exist in the Basidiomycetes, with the added explanation 
that probably certain of the higher forms of fungi that have lost their 
sexual methods of reproduction have been derived from the lower 
