34 



can hardly have existed from the foundation of the world ; they 

 must each have had their period of development, and what has 

 developed through one period of time, may be developed again 

 under similar circumstances. This may require a combination 

 of circumstances acting simultaneously. Each circumstance 

 separately may be in frequent action, but the necessary com- 

 bination may present itself very rarely. A certain temperature 

 and a special pabulum — a product perhaps of a certain stage 

 of putrefaction — may be the first requisites to develope the 

 pathogenic power of the bacterium ; it may then require a 

 special condition of the living animal organism, a lowered 

 vitality, or an enfeebled state of health — the result perlia])s oi 

 the same conditions of air, soil and temperature &c. which have 

 called forth the power of the parasite — before it can find an 

 entrance; and possibly after that its transmission through 

 different species of animals may be necessary before it acquires 

 destructive virulence, and is able, like the bacillus anthracis, to 

 attack sound and healthy tissue. Instances are not rare of 

 vegetable cells possessing poisonous properties at one time and 

 not at another. Nageli has called attention to one of the most 

 striking which is thus described by de Bary : — " The bitter 

 almond tree is poisonous from the amount of amygdalin it 

 contains, though it is not very dangerous to human beings; 

 the sweet almond contains no amygdalin and is not poisonous. 

 The sweet almond tree does not differ specifically from the bitter ; 

 a tree with bitter seeds may be produced from a sweet seed ; 

 bitter and sweet seeds may even be borne on the same tree in 

 flowers and fruits not morphologically distinguishable from each 

 other." What the origin or cause of this difference is, has not 

 yet been discovered, and no explanation can as yet be off"ered ; 



