XVI.] 



THE LAWS OF VARIATION. 203 



are not all really distinct species, but that the same germ 

 will develop into different forms, according to the soil on 

 which it falls. " Thus, no Picccinia but the Puccinia rosoe 

 is found upon rose-bushes, and this is seen nowhere else ; 

 Omygena exigua is said to be never seen but on the hoof of 

 a dead horse ; and Isaria felina has only been observed 

 upon the dung of cats deposited in humid and obscure 

 situations." ^ We can scarcely believe that the air is full 

 of the germs of distinct species of Fungi, of which one 

 never vegetates until it falls on the hoof of a dead horse, 

 and another till it falls on cat's dung in a damp and dark 

 place. It is probable — indeed, in my mind, certain — that 

 among the lower Fungi, as among crystals, the same species 

 assumes different forms according to the medium in which 

 the development takes place. 



Variations of this kind are clearly not of the class of 

 spontaneous variations, for the form is determined by the 

 medium in which the development takes place. It is 

 difficult to say whether they are functionally produced. 

 The inferior Fungi appear to be very susceptible of func- Function- 

 tionally-produced modifications : thus, they are sometimes ^ygg^'mo- 

 developed in liquids, and then they assume very much of difications 

 the character of Algee.^ But it is difficult to see how any ^'^ "^^"'' 

 change of form could make one of those low Fungi which 

 constitute mould better suited to a new habitat ; and I 

 incline to think that the variations by which the same 

 germ of mould develops into different forms, in different 

 situations, is rather analogous to the variations of crystal- 

 line form of which I have spoken than to any result of the 

 laws of habit. 



The power of some of the lower organisms to develop Origin of 

 into different forms in different situations is probably the 



1 Carpenter's Comparative Physiology, p. 214, note. 



^ Ibid. p. 198. A remarkable instance of the change caused by removing 

 a fiingus to a new medium is mentioned in a communication from Mr. 

 Varley to the Microscopical Society of London (Zoologist, 1850, p. 2674). 

 After describing a fungus that often destroys the common house-flj', the 

 report goes on : — " By immersing the fly in water, Mr. Varley found that 

 the mode of growth of the fungus was altered, the heads being no longer 

 produced, and the whole plant becoming long, crooked, and filamentous." 



/ 



