30 Lectures on Bacteria. [^ iv. 



The arthrosporous species, Crenothrix and Beggiatoa, which 

 have been mentioned above, are particularly striking examples 

 of pleomorphous species, if the accounts of them which we 

 possess are correct (see Lecture VIII). But it is in these very 

 forms that the course of development, as we have already pointed 

 out, has not been so completely followed out and so clearly ex- 

 plained as to exclude the possibility, that the apparently irregular 

 pleomorphism is due in these cases to the admixture sometimes 

 of several less pleomorphous species. And even if we accept very 

 extreme statements with regard to Bacteria, it is nevertheless 

 true that the most pleomorphous Bacteria show a very high 

 degree of uniformity when compared with the lower plants 

 mentioned on page 26, such as Botrydium, Hydrodictyon, and 

 many others. 



Any one not familiar with the subjects and investigations in 

 question will be inclined to ask how there can be such a pro- 

 found difference of opinion as that between the negation and 

 affirmation of the existence of species in Bacteria. The answer 

 is, that the difference has its origin in the differences and to 

 some extent in the mistakes in the method of investigation. I 

 do not use the word method here in the customary sense of 

 manual skill and practical contrivances in investigation, but to 

 express the course of procedure in examining and judging of 

 the observed phenomena. 



Species, as is acknowledged and as has been already pointed 

 out above, can only be determined and recognised in and 

 through the course of development, and this consists in the suc- 

 cessive development of forms, one from another. The forms 

 which appear later in the series proceed from the earlier forms, 

 as parts of them, and are therefore at every moment in un- 

 broken continuity with them, even when subsequently separated 

 from them. The proof that they all belong to one and the 

 same course of development can only therefore be established 

 by proving this continuity. The attempt to establish it in any 

 other way, for example, by ever so careful observation of the 



