78 Causes of Disease. 



ments of organs worthless to them in their present form of life, 

 but which were evidently inherited from their free-living ancestors. 

 For example, in lingiiatnla tccnioidcs, a permanent parasite of the 

 nasal fossae of the dog, there are rudimentary feet, probably 

 better dev^eloped in the free-living predecessors, but stunted in these 

 parasitic descendants from lack of use. In the same way may be 

 explained the absence of wings in parasitic insects ; for example, 

 sheep lice have probabl\" developed through atrophy of their 

 wings from disuse of these organs, for there are allied species of 

 insects which are not parasitic and possess wings. On the other 

 hand, gradual adaptation to life upon an animal host leads to the 

 acquirement of new peculiarities, on the part of the parasite, as a 

 transformation of masticating mouth parts into piercing and 

 suctorial organs and the development of special types of organs 

 for attachment and holding. Such adaptations and transforma- 

 tions of structure are of such common occurrence throughout na- 

 ture, even in case of the human body (formation of a horny sole 

 on the back of the foot, originally soft, in cases where from some 

 disease the dorsum of the foot is turned down and walked on ; 

 transformation of the toes into prehensile organs by practice), 

 that, as it is true in the higher animals, there is no wonder that 

 it should occur in these lower invertebrates whose tissues are 

 much more capable of such adaptive growth. The newly ac- 

 quired characteristics become fixed in the succeeding generations 

 because of heredity and because of uniformity in the vital condi- 

 tions about them ; some species which have gradually assumed 

 parasitic nature are finally restricted entirely to a parasitic life 

 and are unable to live in the external world ; others pass only a 

 part of their lives, a definite developmental stage, as parasites, and 

 for the rest of their time live independently. There are certain 

 filarial worms which ordinarily live in moist earth, but which now 

 and again, as necessity arises, become parasitic. As the moisture 

 disappears these worm retreat into depressions where the moisture 

 is longest retained, but should a snail or earth-worm happen in 

 their way they pick out the cavities of these animals and creep 

 into them. In protracted drouths they are parasitic for a corre- 

 spondingly long period, and it has been observed that in such 

 parasitic existence they attain unusual size and produce a larger 

 number of ova than their free-living fellows (Braun, Heller). It 

 may be that the young brood continue in the snail and become 

 completely transformed into parasites, but the original ones, when 



