CONSTANCY AND MUTABILITY OF FUNGI. 



external conditions than the other more normal examples, 

 or when the cultivator intentionally selects the individuals 

 which show a certain kind of abnormality, and only employs 

 these for further cultivation. 



In this way, Darwin's hypothesis seeks to explain the 

 origin of varieties, species, and genera. But it is im- 

 portant and necessary for the definition of a well-marked 

 species that the new properties should be relatively con- 

 stant, and not undergo alteration, under the influence of 

 The varieties varying external conditions. As a matter of fact it seems 

 duced by r to be impossible, by selecting the external conditions, to 

 external con- produce other species at will ; in experiments per- 

 formed with this aim the plants always remain the 

 same, they at most undergo modifications and degenerate, 

 but they do not acquire any constant and hereditary 

 abnormalities, provided that there is no tendency to 

 abnormal growth in the plants themselves. The origin 

 of all varieties may be referred to a tendency in the 

 plants towards variation which is peculiar to them and 

 cannot as yet be explained. This tendency is of very 

 varying intensity according to the species of plant; some 

 form varieties extremely readily, others extremely seldom, 

 and the variations occur quite as well when the plants 

 are all kept under conditions which are as much as 

 possible the same as when they are subjected to different 

 influences. It is only the vital power and the power of 

 development of the varying plants that depends on the 

 sum total of the external conditions. Of the most 

 marked influence on the tendency to variation is the 

 sexual union of different individuals ; where such a union 

 occurs variations are usually formed very plentifully. 

 But even without sexual processes the tendency of many 

 plants to form new varieties is very great. 



Conclusions If now we attempt from these views as to the origin 

 ap pHeTtotL and characteristics of the species of the higher plants, 



lower fungi, w hich have been put forward more especially by Nageli, 

 to obtain a standpoint to explain corresponding variations 

 in the lower fungi, we must assume that in these the 

 formation of modifications, varieties, and species must 

 as a whole occur in a similar manner. Certain altera- 



