42 Proceedings of Indiana Academy of Science 
science of bacteriology and with it the proofs that life does not arise 
spontaneously, and finally the formulation of the modern theories of 
heredity. Thus the old theories have been proven untenable so that now 
the period from generation to generation represents a definite sequence 
or a series of steps in a definite life-cycle. 
The term “parasite”, now one of the common terms in_ biological 
literature, is so difficult to define that it seems almost impossible to 
have two workers agree without a series of qualifications of its meaning. 
Megnin has defined parasites as those which live at the expense of others 
which are living. The definition recognizes a symbiotic relationship be- 
tween the parasite and its host, upon which or within which it may 
maintain its existence for a shorter or longer time, possibly only for 
an occasional visit or for the whole of a lifetime. 
Whatever the relationship to its host the parasite, if it is successful, 
lives and grows and in turn reproduces its kind, but to do so it must 
solve the problems of life. These may be serious ones, and the hazards 
of gaining a foothold and maintaining itself and reproducing are out 
of all proportion to those with which man, his domesticated animals and 
most other free-living forms must cope. 
The parasite may be a permanent or a temporary resident upon 
the outside of a host that mingles little or much with others of its 
kind; it may live within the digestive tract or kidneys, liver, lungs or 
in the circulatory apparatus, within which it may move with more or 
less ease but from which escape to a new host becomes more difficult; 
it may live within the tissues from which escape seems even less certain, 
unless the host is eaten by some predaceous form, or its carcass is de- 
voured by some scavenger. 
If it is on the outside of a host how does the parasite maintain 
its place or avoid dislodgment? 
How does it maintain itself inside the body without being 
strangled, if respiration is necessary ? 
How does it maintain itself in a digestive tract in which it 
encounters the inhospitable digestive enzymes that act upon other 
substances like those of its own body? 
How does it perform the fundamental functions of life, as re- 
producing itself, ete.? 
If it or its young find the way to the soil or water how are the 
new conditions withstood ? 
If neither it nor its eggs or the young can escape from the 
host, what of the future? 
What chance do the progeny have of securing a foothold and 
what difficulties do they encounter, or how do the progeny come to 
find a similar or suitable host? 
Above all, how did the particular parasite come to live as it 
does? 
These are some of the problems that the successful parasite solves 
when it maintains itself and reproduces its kind. 
