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tion I wish to consider briefly the causes which probably led to the 
assumption of the parasitic habit. 
There are three advantages in a parasitic life such as is led by the 
Sarcoptide over the independent life of their nearest allies. First, it 
is an escape from competition with forms better endowed for maintain- 
ing themselves as free-living organisms. Second, it is an escape from 
the host of predaceous insect enemies to whose attacks the roaming 
mites are subject. Third, it is an advantage of afood-supply not subject 
to the accidents that must often affect the supply available to the free- 
living mites. 
With an advantage ever so slight in favor of a parasitic life, the 
stress of competition is such that individuals will be compelled to as- 
sume it, or will voluntarily do so. Itis asif organic nature were plastic, 
and were constantly subjected to a tremendous pressure which forced 
it into all available unoccupied space. The necessity which leads to the 
struggle for existence is such pressure; the plasticity consists in the 
inherent tendency to vary. 
Take for illustration a species the food of which is dead vegetable 
matter. There will come at times to individuals a scarcity of this kind 
of food, and hunger may force them to devour animal refuse, or through 
some circumstance such animal products may constantly occur among 
the normal food of the species, and certain individuals through physio- 
logical or morphological peculiarities may gradually acquire a fondness 
for such food; thenif a period of scarcity of vegetable food comes. 
these mixed feeders have the advantage and increase over their fel- 
lows. The taste for animal food becomes fixed by natural selection, 
and eventually we may have a variety or a species which feeds exclu- 
sively on animal food. 
Supposing such species of mite to occur among dead leaves, and that 
the source of the animal food is refuse from the prey of some carnivor- 
ous mammal, as a wolf or fox. The accumulation of fragments of fowls 
and small mammals about the haunts of such carnivores would furnish 
an abundant supply of animal food. Among the waste from the food 
would be considerable waste also from the body of the wolf or fox, 
such as worn hair, fragments of the epidermis, and the like. If a 
pressure for food came to mites dependent on this supply it is easy to 
imagine them resorting to the bodies of the sleeping mammals them-~- 
selves to browse upon the loosened epidermis, as certain mites are known 
to do, instead of collecting it as formerly from the ground. 
This habit of resorting to the skins of living mammals would be only 
temporary at first, and the species would still pass most of its life 
among refuse or in the ground. But in time forms would arise better 
fitted for clinging to the skin of mammals, better able to make their 
way among the pelage, perhaps able to remain there at all times, dis- 
placing the less favored mites, and by selection becoming adapted to 
a life on the bodies of mammalia, though still feeding on the dead 
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