Life Among Parasitic Animals 43 
The last of these questions, “Above all, how did the particular 
parasite come to live as it does?” may be considered first, in a gen- 
eral way, as it applies to a few of the permanent ectoparasites. From 
the investigations of others and from my own observations I have been 
able to obtain a large volume of data on various aspects presented in 
different states of symbiosis which have been included under the term 
parasitism. 
To each of us—though with certain reservations—it has been a 
matter of common observation that the mosquito, the blood-sucking fly, 
or the bed-bug and flea, or a tick, may make an occasional levy for blood, 
but that any warm-blooded animal may serve equally well as a temporary 
host, only long enough to secure their fill, or until they are interrupted 
in the act. The length and strength of the piercing beak here seems to 
guarantee an ample supply of food from any warm-blooded animal whose 
skin is not too thick to be penetrated, so the food problem is not a diffi- 
cult one. Sooner or later they lay their eggs which develop apart from 
the host, and in time the young of only one of these return for the food 
that is necessary to maintain a livelihood, and for them the food ques- 
tion is simple, for they share the same bed with their host. The 
progeny of the flea and the mosquito forage in a free environment but 
the ticks, numbering from two thousand to ten thousand from a single 
mother, crawl up stems and blades of grass and there wait. If a new, 
warm-blooded animal passes, or if they chance to attach themselves to 
a favorable host, they may gain a meal and live, but the hazards are 
great, and on the average there are no more ticks in one season than 
in another, which means that a little tick has scarcely one two-thou- 
sandth, or even one ten-thousandth of a chance of reaching maturity. 
As one leafs over a series of volumes and reprints he may observe 
little flat-bodied, wingless book-lice that have been disturbed in their 
meal upon the glue of the bindings, and in them he may recognize the 
same characteristics of structure that occur in a host of forms that 
live as parasites, permanent parasites, on our domestic poultry and on 
our wild birds. Other relatives of these free-living book-lice are found 
upon the bark of trees, and in some manner they may have found their 
way to the bodies of the ancestors of our present-day birds. The whole 
organization and structure is so similar that we have good reason to 
conlude that the bird-lice, the Mallophaga, fifteen hundred different 
species, have descended from the ancestors from which the book-lice 
have come. The food which they now take consists of the small frag- 
ments of feathers and dead skin, instead of the glue or other animal 
or plant products of their free-living relatives. 
The eggs of the Mallophaga are glued to the feathers, or to the 
hairs, for about one hundred of the fifteen hundred known species have 
taken up their abode on mammals, by a clasping action of the posterior 
end of the abdomen of the female. The young feed from birth upon 
hairs or feathers and free scales of skin as the parents do, and neither 
they nor their parents leave the body of the host except under unusual 
circumstances. It is only very rarely that straggling lice are found on 
the perches, or the roosting places on seaside rocks from which scores 
