Inter-Relations of Living Creatures 665 



in some other countries is able to utilise another species of water- 

 snail. When we examine the food-canal of a mammal, bird, or fish 

 that has not previously been studied in this connection we often 

 find a new tapeworm or threadworm. We wonder how far this 

 illustrates the r6le of isolation in assisting the formation of new 

 species. Just as there is an Orkney Vole and a St. Kilda Wren, 

 and a distinct species of land-snail in each of the sharply isolated 

 valleys in Hawaii, so there are different species of tapeworms, 

 flukes, and threadworms in diverse hosts. We wonder whether 

 the apparently different species of parasites may not be to some 

 extent the same species, slightly modified by the differences in 

 surroundings and food ! Here there is opportunity for important 

 experiment. 



What is it exactly that parasites do to their hosts? Some 

 absorb a good deal of digested food from the intestines ; some per- 

 forate the wall of the food-canal; some cause inflammation by 

 vitiating the surrounding tissues; some block passages, even 

 blood-vessels; the mite of so-called Isle of Wight bee-disease 

 blocks certain air-tubes and cuts off air from the muscles, besides 

 feeding on the bees' blood ; the sturdieworm of the sheep presses 

 upon the brain or the spinal cord, causing serious locomotor de- 

 rangements; some tapeworms and threadworms secrete a toxin; 

 some peculiar crustacean parasites, e.g., Sacculina, destroy the 

 reproductive organs of male crabs. In short, the influence of 

 parasites is very manifold. 



6 



The Romance of Parasitism 



There is something repugnant in most parasites; we cannot 

 look at them without some ethical recoil. We know that they are 

 creatures which do not fend for themselves. Moreover, many of 

 them are far from beautiful, which may be to some extent the 

 stigma of their degeneracy. A sluggish pampered animal is not 



