§ IV. J TAe question of species. 2 7 



another. According to the one view their case is the same as 

 that of all organisms other than Bacteria, that is, of all other 

 plants and animals ; like these they are distinguished into 

 species. This was accepted as a matter of course by the earlier 

 observers from the first discovery of the Bacteria by Leeuwen- 

 hoek (3), to the more careful and extended observations of these 

 organisms which was undertaken by Ferdinand Cohn (4) at the 

 beginning of the period from i860 to 1870. Cohn, following 

 in the steps of his predecessors, especially Ehrenberg (5), en- 

 deavoured to give a general view and classification of the forms 

 which had become known to himself and others. It was im- 

 portant to arrange the material in hand and waiting further ela- 

 boration in some provisional manner, and to do this it was 

 either allowable or necessary to start from the assumption, 

 which certainly required to be proved, that a species was always 

 characterised by a definite form, as is the case with the above- 

 mentioned comparatively monomorphous kinds. The species 

 were therefore distinguished by their shape and growth-form, with 

 some help from their effects on the substratum, and then further 

 classified. The names Coccus, Spirillum, Spirochaete, &c., applied 

 above to growth-forms corresponding to such terms as tree and 

 shrub, were used as names for definite natural genera like birch, 

 chestnut, &c. ; such genera we may accordingly therefore term 

 form-genera. Whether these form-genera and form-species did 

 or did not really coincide in all points with natural genera and 

 real natural species, was expressly left undecided by Cohn and 

 reserved for further investigation. 



Cohn's view as expressed in his provisional classification was 

 opposed by other writers, who went so far as to deny that there 

 were any species of Bacteria. They considered that the ob- 

 served forms proceeded alternately from one another, the one 

 being convertible into the other with a change in the conditions 

 of life, and that this change might be accompanied by a corre- 

 sponding change in the effects on the substratum, though this 

 point does not, strictly speaking, belong to the subject which we 



