io6 THE SCIENCE OF LIFE. 



11. The difficulty of determining what forms are to be 

 regarded as species, and what as mere varieties, finds 

 many illustrations in the class of Fungi. We know but 

 little of the influence of external conditions in modifying 

 forms, and the forms of fungi are so exceedingly unsta- 

 ble that the best observers are often at a loss. Yet this 

 variability is only one of degree, since all living beings are 

 more or less subject to modifications of form by external 

 influences. It is this variability which has rendered the 

 Darwinian hypothesis of evolution by " the survival of 

 the fittest " so plausible a theory. But notwithstanding 

 this capability of modification, there is still a certain fun- 

 damental and specific type for each assemblage of forms, 

 and the amount of variability is strictly limited. 



12. Many diseases of plants and animals are associated 

 with the growth of Fungi. The " mildew " (Puccinid) 

 and " rust " (Uredo) of wheat, etc., the potato blight, 

 (Peronospora,) the disease in Silk-worms called Muscar-- 

 dine, (Botrytis,) the false membrane in diphtheria, the 

 white patches in aphthae, or thrush, and many skin af- 

 fections, afford examples. Pyaemia is supposed to result 

 from bacteria in the blood, and many epidemic diseases 

 have been ascribed to similar origin. The prevalence of 

 atmospheric changes, however, and variations in external 

 conditions, as light, heat, moisture, etc., have much to 

 do in predisposing both animal and vegetable tissues to 

 disease, and in producing epidemics. Since the office 

 of Fungi is to remove decaying or effete organic matter, 

 we must discriminate between those diseased conditions 

 which provide a habitat for fungi and the effects pro- 

 duced by the fungi themselves 



