PARASITES 185 
parasitism is very widely spread, and there is probably no animal 
which does not unwillingly entertain unwelcome guests that make 
no return for services rendered. As De. Morgan sings (in The 
Budget of Paradoxes) :— 
“Great fleas have little fleas upon their backs to bite ’em, 
And little fleas have lesser fleas, and so ad tnjinitum; 
And the great fleas themselves, in turn, have greater fleas to go on, 
Whilst these again have greater still, and greater still, and so on”. 
The general progress of evolution has been from the less to 
the more specialized as a result of adaptation to increasingly 
complex surroundings, but to this parasites constitute a striking 
exception. Free quarters and free rations having been provided 
for them, they have taken but little part in the active struggle for 
existence, and well illustrate the principle of Degeneration. They 
are on the down-grade, adapting themselves to comparatively 
simple conditions. Hence we find that complex organs of diges- 
tion, circulation, respiration, and locomotion, together with nervous 
system and sense-organs, have undergone more or less reduction 
in thoroughgoing parasites, though, on the other hand, they have 
frequently developed special piercing, sucking, and adhesive struc- 
tures, enabling them to exploit their living food-supply, and to 
maintain their position. The great danger attending this par- 
ticular mode of life is constituted by the smallness of the chance 
of transfer from one host to another. In the more helpless forms 
this difficulty is often met by the practice of living in two or more 
different hosts which eat or prey upon one another; the adult 
egg-producing stage, being the most important, is commonly 
associated with the strongest and most highly organized of these, 
the so-called “final host”. The biological relations between the 
successive living refuges is always such as to maintain most surely 
“the vicious circle of parasitism”. Even more important is the 
immense fecundity of parasites, a necessary provision, for the 
chances of survival are extremely small. Leuckart calculated, 
for example, that any one egg of a tape-worm has only one chance 
in some 8 3,000,000 of giving rise to an adult. 
What is called Brood Parasitism, where an animal shirks the 
duty of bringing up its own young, will be considered in this 
section, although it is by no means the same thing as true 
parasitism. 
