26 Lectures on Bacteria. [ iv. 



partly from the effect of external causes which are known and 

 may be varied at pleasure in our experiments, partly from 

 internal causes which cannot at present be analysed. The 

 white mulberry-tree, for example, in contrast to the horse- 

 chestnut just mentioned produces foliage-leaves very unlike each 

 other and with no certain rule of succession, some simply cor- 

 date, others deeply notched and lobed. We should not recog- 

 nise the species by a leaf of the latter kind, if we had before 

 only happened to have seen the cordate leaves. This occurs 

 frequently and to a still greater extent in the lower plants, 

 though they need by no means belong, like the Bacteria, to the 

 simplest and smallest forms. Many of the larger Fungi, for ex- 

 ample, the forms of Mucor,and green Algae, such as Hydrodictyon 

 and the remarkably pleomorphous Botrydium granulatum, exhibit 

 phenomena of this kind in a very striking manner, especially 

 when it further happens, as it often does happen in similar 

 plants, that the successive members of the development do 

 not continue in prolonged connection with each other, like the 

 leaves of the mulberry, but separate and vegetate apart from 

 one another. In this case if we happen to find the objects 

 separate and alone, and are accustomed from our experience 

 of the chestnut always to judge of a species by the individual 

 form, we fall into mistakes such as the history of botanical study 

 can supply in great abundance. But if we observe how each 

 form developes and how it originated, we perceive that they 

 have all the same course, the same origin, and the same return 

 to similar beginnings, or as we may say, conclusions of the 

 development. 



The pleomorphous species therefore differ from the relatively 

 monomorphous species only in the greater number of forms 

 and in the greater amount of differentiation in the course of 

 development; the qualities of the species are apportioned in 

 equal measure in the one as in the other. 



As regards then the species of Bacteria two views have been 

 promulgated, which in their extreme form differ much from one 



