ADJUSTMENT OF ORGANISMS TO ENVIROXMEXT 397 



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The cowbird, relieved of the care of her young, has lost her 

 nesting instincts. The Indian pipe (fig. 233) attached to 

 the roots of trees whence it can draw manufactured carbon 

 products, has lost its green color and its leaves. Sacculina, 



that famous illus- 

 tration of the degen- 

 eracy that results 

 from the parasitic 

 habit, living in the 

 the perfect nurture 

 and shelter afforded 

 by its crab host, 

 has lost all those 

 structures and 

 capacities by which 

 we recognize its free 

 living kindred. 



In general it 

 may be said that in 

 proportion as the 

 conditions of living 

 become simple, easy 

 and secure, the 

 parasite comes to 

 lack those organs 

 and faculties necessary to meet hard conditions, in battling 

 with which they were developed. This loss is not the result 

 of the parasitic habit, but of the sheltered life that goes with 

 it. The series of insect larvae we have used to illustrate 

 metamorphosis, excellently illustrates degeneracy also, 

 though none of the larvse used was parasitic. It would not 

 be difficult to select parasitic insect larvae, that would 

 constitute parallel degeneration series. It seems clear that, 

 as in the individual, so in the long run in the race, it is 

 effort that builds; disuse leads to degeneration. 



Fig. 232. A parasitized moth larva on blue grass 

 top: some of its parasites ha\e spun their cocoons 

 beside it, others on the grass blade above; b, 

 shows an easy miethod of getting the adult para- 

 sites from the cocoons. 



