38 Lectures on Bacteria. [^ v. 



we may at once assume that their origin is the same as that 

 of other plants, that is, that the Bacteria existing at any given 

 time have sprung from beginnings which proceeded from in- 

 dividuals of the same species, and experience shows that this 

 is really the case. These beginnings may be spores or any 

 other cells capable of life ; we shall here usually call them 

 germs. 



The germs of living beings, especially plants, are extra- 

 ordinarily numerous. They may be said to cover the surface 

 of the earth and the bottom of the waters with an infinite 

 profusion of mingled forms. The number of plants observed 

 in the developed state gives no idea or only a very imperfect 

 idea of this fact, because a much larger number of germs is in 

 all cases produced from a single plant than can arrive at their 

 full development in the space at their command, which is in 

 fact always limited. The smaller the organisms are, the greater 

 advantages they enjoy as a general rule caeteris paribus for the 

 production and distribution of their germs, for it is so much 

 easier for them to find space and a sufficient quantity of food for 

 their development and for the production of new germs; the 

 mechanical conditions for the transport of the germs from place 

 to place are also more favourable in proportion as the volume 

 and mass are diminished. For these reasons the number and 

 distribution of the germs of lower microscopic organisms, espe- 

 cially in the vegetable world, must seem astonishingly great to 

 any one who is unprepared for the facts. If spring-water is 

 allowed to stand in a glass, it becomes green in time from the 

 growth of small Algae, whose germs were present in the water 

 before it was placed in the glass or have been carried there with 

 particles of dust. If a small piece of moistened bread is placed 

 in the water a growth of mould soon makes its appearance, pro- 

 ceeding from germs of Mould-fungi. Some time since I made 

 researches with a different object into the Saprolegnieae, a 

 group of rather large Fungi consisting of about two dozen 

 well-known species, which grow in water on the bodies of 



