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minute, and probably of a lower type, I shall ask you to note 

 for a few moments one or two simple experiments, which, al- 

 though made on non-living material, will prepare the way for 

 further study. 



If we take any kind of organic matter — whether animal or 

 vegetable is of no consequence — and place it in water and ex- 

 pose it to the air at a moderate temperature we shall see after 

 a day or two a very thin pellicle floating on the surface. If 

 we examine a small portion of this under a moderately high 

 power we shall find that for the most part it consists of minute 

 spherical or rod-shaped bodies, many of which from their 

 peculiar gliding or undulating movements we know to be 

 alive. If we watch them for a sufficient time we shall get 

 further evidence of their vitality by seeing them grow, divide, 

 and multiply. If we repeat our experiment with different 

 materials and under varied conditions we shall find that, al- 

 though the living bodies that grow in our solutions have a 

 strong family likeness, there are differences in shape and in 

 mode of motion which enable us to attempt some form of 

 classification. Some of them look like simple round points ; 

 some are more rod-shaped ; while others, again, are long and 

 narrow. Some appear to be quite motionless ; others glide 

 slowly along from one side of the field to the other ; while 

 others, still, will be seen to wriggle with a spiral corkscrew 

 kind of motion from point to point. It's a busy world we are 

 looking at. AVe shall see, further, that, although all of them 

 are minute, there are considerable differences in size, some of 

 them being many times larger than others. If we try to obtain 

 with the highest powers at our disposal a more intimate know- 

 ledge of their organization we find them exceedingly simple in 

 structure — in fact, they appear to be nothing more than 

 minute masses of protoplasm, which are probably enclosed in. 

 a cell membrane. For a long time they were supposed to be- 

 long to the animal kingdom, and their movements gave some 

 support to this view ; but in later times most observers place 

 them in the lower orders of plants, some botanists classifying 

 them as algse, while others with perhaps more reason assign 

 them to the fungi. In the French school they are commonly 

 known as les microbes. In England they are more frequently 

 included under the common term Bacteria. In one form or 

 another they exist everywhere — in the air, in the water, and 

 in the soil. As to their origin, there has been much conten- 

 tion. Some observers have gone so far as to maintain that 

 they originate by self-generation, while others more cautious 

 and more numerous concur with Pasteur and Tyndall that 

 wherever we find them they owe their existence to previously 

 existing germs, which in different modes have found their way 

 into the fluid or structures in which they are discovered. 



