almost immediately. An equilibrium may not 
be reached until long after the fecal pad has 
disappeared. What has happened to the para- 
sites? Many, but usually not all, have died or 
been destroyed. Darwinism in a teacup! Some- 
times predaceous fungi have been at work, 
here trapping, there entangling, and elsewhere, 
like the vulture of Prometheus, gnawing at the 
vitals of the young parasites (13, 14). It is 
quite likely that some parasites succumb to 
actual infections of bacteria or viruses quite 
specific for them. 
Recently scientists of the U.S. Dept. of Agric., 
Agricultural Research Service found that asca- 
rid worms in the gut of swine may be infected 
with an unidentified species of Pseudomonas, 
and that as the evidence of such infections in- 
creased, the population of ascarids inthe animal 
was smaller than otherwise might be expected 
(15). The pig probably acquired the pseudomon- 
ads from the soil, although the infection of the 
worms probably occurred inthe definitive host, 
Scientists at the Beltsvillé Parasitological 
Laboratory have identified from fecal pads of 
cattle, and the soil beneath or nearby, orga- 
nisms representing almost a dozen phyla of 
the animal kingdom, quite in addition to the 
myriads of plant organisms. Not all have an 
adverse effect on the spread of parasites. For 
example, the larvae of some parasitic 
helminths migrate onto and take advantage of 
the remarkable spore-scattering mechanism of 
fungi of the genus Pilobolus. In one instance, 
studied and recorded by Robinson in England 
(16), the larvae are those of Dictyocaulus 
viviparus, the cattle lungworm. Following the 
explosive rupture of the sporangium, larvae 
were recovered from dishes placed as much 
as 10 feet from the fecal pad on which the 
fungus was growing. 
Scientists with the U.S. Department of Agri- 
culture in Georgia have recently found the 
same mechanism to operate in the dissemi- 
nation of larvae of certain other parasitic 
nematodes (17), It is probably very common. 
It affords the young parasites at least four 
advantages; (1) It removes them from the area 
of extremely severe competition; (2) it in- 
creases by a hundredfold or more the grazing 
area over which they are dispersed; (3) it 
further increases their chances of being in- 
gested by depositing them on relatively clean 
forage, as opposed to having to await their fate 
114 
at sites that grazing animals usually avoid; 
and (4) it accomplishes all these with minimal 
expenditure of the precious food reserves so 
desperately needed for later use. 
To be able to curtail such a natural device as 
this should aid inthe control of some parasites. 
Among the many inhabitants or frequent 
visitors of communities, such as we have noted, 
are several that serve as intermediate hosts, 
or vectors of other types, for many of our most 
common parasites, Methods of biological con- 
trol of any such vectors might contribute toward 
the control of some parasites. Flukes, for 
example, require appropriate snails as inter- 
mediate hosts. Biological control of snails may 
vary from the simple expedient of rearing 
ducks or geese in the pastures ranged by 
susceptible vertebrate hosts to the more re- 
fined, but possibly no more effective, device 
of replacing populations of unwanted snails 
with species not known to be harmful (18, 19), 
or the still more sophisticated method of using 
the larvae of marsh flies tokill the snails (20). 
As I have previously indicated, if a parasite 
is controlled by the exclusion or elimination 
of an important or essential vector, this by my 
definition is biological control, regardless of 
the nature of the means used against the 
vector. Blackhead, or histomoniasis, a disease 
of gallinaceous birds, almost destroyed the 
turkey industry in America half a century ago, 
and has become troublesome in chickens of 
some recently developed breeds. It is caused 
by a protozoan, Histomonas meleagridis, 
mentioned earlier as a relatively immutable 
parasite. Itself a very fragile organism, it gets 
from bird to bird through the sturdy eggs of 
a small nematode, which also resides in the 
ceca of gallinaceous birds, and is Heterakis 
gallinarum, the common cecal worm. But 
sturdy as these cecal worm eggs may be and 
able to survive in soil for 2 or more years 
under appropriate conditions (21, 22), the eggs 
themselves lack the means of getting back into 
the bird, except as they may be picked up by 
accident. This is a precarious arrangement 
for an organism that produces eggs in such 
small numbers, as by the hundreds (23), to 
infect creatures that in the evolutionary past 
were never crowded and probably migrated to 
new feeding grounds before the eggs voided in 
a given area had become infective through 
embryonation. 

