184 



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 

 Sarcoptidae 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 j)redaceous insect enemies to whose attacks the roaming- 

 mites are subject. Third, it is an advantage of a food-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 comijetitiou is such that individuals will be compelled to as- 

 sume it, or will voluntarily do so. It is as if 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 veg'etable 

 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 i)roducts 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 ; then if 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 suj^ply 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 i^elage, perhaps able to reuiain 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 



