304 
Journal of Agricultural Research 
Vol. I, No. 4 
lar parasite has of successfully reproducing itself from season to season, 
and consequently the more general and severe will be its injury over 
that area. Thus, a disease occurring occasionally or with but slight 
injury upon more or less isolated host individuals may under conditions 
of close orchard planting assume an entirely different aspect, becoming 
more nearly seasonal in its occurrence and causing a much greater per¬ 
centage of injury. A large part of the assumed difference in injury by a 
disease under native and under orchard conditions is, however, often 
merely psychological. In orchard culture the ideal sought is a thrifty 
growth and abundance of high-grade fruit for every tree planted. Any 
deviation from this ideal is quickly noted by the grower; whereas little 
consideration is given to the facts that under native conditions large 
numbers of individuals succumb to disease for every one that persists 
and reaches maturity and that careful observations and comparisons are 
seldom made with those which do reach maturity. 
Nevertheless, the general fact remains that well-known diseases are 
often more destructive under orchard conditions. Further than this, 
diseases of hitherto unknown occurrence upon a particular host may 
suddenly make their appearance. Some of these may have been present 
but previously unnoticed, while others may be actually new to the host. 
They are often brought to a locality with the introduction of new plants, 
and with the widening of the range of a host the diseases of related plants 
will be encountered sooner or later. Furthermore, a parasite is often 
more destructive when brought to a new locality, either because of the 
absence of its former enemies or because of other conditions more favor¬ 
able to its growth and reproduction in the new environment. 
It has long been known that where a considerable number of plants 
or animals are exposed in a similar way to the attacks of a parasitic 
disease more or less difference will be noted in their behavior toward 
the disease. In many cases some individuals will be found which seem 
to be entirely immune, others which are very susceptible to attack, and 
still others with varying grades of immunity or susceptibility between 
two extremes. In localities favorable to the growth and spread of a 
disease this condition works for the general benefit of the species attacked. 
Those individuals least susceptible to injury will be most successful in 
reproducing themselves, and thus a more or less immune race will be 
developed. On the contrary, if a race has arisen amid conditions un¬ 
favorable to the development of a particular disease, or in its entire 
absence, growth and reproduction will have taken place with little or 
no relation to the disease. If such a race is exposed to the disease, it 
is probable that a large percentage of its individuals will be found to be 
susceptible. 
These relations between host and parasite, though only a few among 
many, may at least serve to indicate the extreme complexity of all prob- 
