GREAT STEPS IN EVOLUTION 105 



and his liver changed from its natural brown 

 almost into the likeness of a tangle of white 

 worsted, of which each thread is a tape- 

 worm. Neither frog nor lizard, serpent nor 

 bird, escapes; indeed birds are peculiar suf- 

 ferers, witness the too common " gapes " of 

 poultry, a choking of the windpipe by thread- 

 worms, and the numerous parasitic worms 

 which Mr. Shipley's labours have discovered 

 in the well-nigh sacred grouse. Of ticks the 

 shepherd is only beginning to know the full 

 dangers, as of fleas man himself. Apart 

 from bacterial and protozoan pests, as yet 

 beyond counting, man is debited by the 

 parasitologist with at least sixty species ; some 

 reckon twice as many. The amazingly varied 

 methods of Nature for the diffusion of para- 

 sites are among the very strangest disen- 

 tanglements of the web of life, but into the 

 stories of these adventures in search of mis- 

 chief we cannot enter here : enough if we 

 note the stupendous rate of multiplication 

 by which the many chances against finding 

 the proper host are constantly met; thus 

 the common tape-worm of man has been 

 calculated to produce eighty-five million ova 

 during its two years' existence. 



It is an interesting inquiry whether the 

 large numbers of so-called species of thread- 



