582 MAN AS A CONQUEROR 



One of the most serious factors against which man has to fight has 

 recently been called to the attention of scientists by Professor E. C. 

 Stackman of the University of Minnesota, and that is the rapid 

 appearance of new strains of harmful fungi. A single reproductive 

 cell of a grain smut was isolated and grown under laboratory condi- 

 tions. In a relatively short time 112 distinct physiological strains 

 were produced from the original plant. This means that under nat- 

 ural conditions there are new strains constantly arising, that will in 

 time attack new crops as they are planted, some living on varieties 

 of wheat, others on oats, barley, or rye. In other words, nature is 

 constantly at work producing new varieties, either through muta- 

 tion or through sexual crossing of existing varieties, thus forming 

 hybrids which are different from the original parents and which have 

 the possibilities of attacking different grains from those their parents 

 live upon. It looks as if man was less than one jump ahead of such 

 plant parasites. 



Harm Done by Animals 



It is not the purpose of these pages to do more than call attention 

 to some of the animals harmful to man, but we should note that 

 some of the most dreaded diseases, such as rabies, malaria, sleeping 

 sickness, and amebic dysentery, are laid at the door of the protozoa. 

 Among the echinoderms, starfish do much damage to shellfish and 

 thousands are dredged up and destroyed each year by oystermen. 

 Cestodes are parasitic in food animals such as cattle, swine, and 

 fishes, and from these hosts may infect man. The class Trematoda 

 also includes many parasitic flukes, some of which may infect man. 

 The Nemathelminthes include the hookworm (Necator) and Trichina 

 as well as the Filaria, which sometimes causes elephantiasis. Para- 

 sitic worms also destroy annually large numbers of fishe?, birds, and 

 mammals used as food. Among the mollusca that do harm are the 

 whelks which destroy other edible molluscs, and the shipworm 

 {Teredo) that destroys submerged timber, such as the piles of wharfs 

 and the hulls of vessels. Of crustaceans, crayfish may become a 

 serious pest to cotton raisers by destroying young cotton plants. 

 A few poisonous spiders exist, such as the notorious "black widow" 

 and the tarantula. The ticks are of much importance because of 

 their parasitic habits and the fact that they carry other parasites, 

 such as the protozoan that causes Texas cattle-fever. 



It is the insects, however, that must rank highest as man's com- 



