32 Lectures on Bacteria. [$ iv. 



organisms, such as Algae and Fungi, both those of the larger 

 kinds and the microscopically minute. The separate species 

 were imperfectly known, and different ones were brought into 

 genetic connection with one another, because observation of 

 continuity was omitted or imperfectly made, and in its place was 

 substituted the observation of the succession in time of forms 

 at the same spot, or the comparison of them as they made their 

 appearance there together. 



The smaller and simpler the forms, the greater certainly is 

 the difficulty of satisfying our logical demand, and the greater 

 the attention which must be paid to it. In small forms consist- 

 ing of isolated cells of no very marked shape, such as some of 

 the lower Fungi and the Bacteria, we must observe carefully 

 whether the sowing contains the germs of a single species or of 

 several mixed together. The latter is very frequently the case, 

 as experience shows. Various species often occur together and 

 mixed with one another at the spots from which the material 

 for the observation was obtained ; during the investigation forms 

 not desired, ' unbidden guests/ may find their way with par- 

 ticles of dust into the material, and even when we are dealing 

 with apparently quite pure material, a small quantity of micro- 

 scopic weeds, as we may say in this case also, may be mingled 

 with it. 



If every thing in the mixture grows at an equal rate, the 

 different species may be kept distinct with comparative ease, and 

 the character of the mixture is clearly understood. But the state 

 of things may be different from this, and experience shows that 

 it often is different. The one species develops vigorously under 

 the existing conditions, the other feebly or not at all ; the more 

 successful species gains the upper hand of the less successful, 

 dispossessing it and even entirely destroying it. Further ex- 

 amination shows that in some cases a weed has grown up in 

 place of the wheat. This may very easily happen. We shall 

 see further on that some Bacteria, for example, double the num- 

 ber of their cells under favourable conditions in less than an 



