CONSTANCY AND MUTABILITY OF FUNGI. 679 



that they can be of use for the formation of distinct 

 varieties. Characters which readily undergo alteration 

 under the influence of all sorts of external conditions, 

 which are readily lost and acquired, are as little suitable 

 for characterising varieties as varying differences in 

 form. 



As a matter of fact we have, in the production of 

 ferments, in the formation of pigment, in the setting 

 up of fermentation and disease, and in the whole mode 

 in which the nutrient substrata are assimilated and 

 broken up by the bacteria, such hereditary and cha- 

 racteristic physiological attributes of the species of 

 bacteria. 



It has been already mentioned above (p. 563) that Variability 

 the sum total of the external conditions of existence of varying 1 



exerts a very marked influence on the quality of the pro- exte al 

 ducts of tissue change, and that by abnormal external 

 conditions every individual phase of the characteristic 

 vital phenomena may be abolished ; but, nevertheless, 

 these variations in physiological characters follow defi- 

 nite lines and do not go beyond the characters of the 

 species in question. In correspondence with this fact 

 those vital phenomena which are constant under certain 

 nutrient conditions can be very readily employed as 

 means of distinction ; and the loss of the individual 

 characters as the result of definite external conditions 

 only serves as a means of increasing the number of 

 characteristics of the species. 



Further, as in the case of other organisms, so also Alteration by 

 probably in the case of bacteria, they may become accli- tioV. 

 matised to abnormal conditions of existence; for example, 

 to an excess of salts, to a different reaction of the nutrient 

 medium, to temperature, &c. It is conceivable that 

 the same abnormal conditions when suddenly employed 

 cause cessation of development, while when gradually 

 introduced they still permit the exercise of the vital 

 functions. More accurate facts, however, are still want- 

 ing as to the behaviour of bacteria under such conditions, 

 but it is a priori probable, from what we know of the 

 behaviour of the higher organisms, that even under 



