366 Transactions of the Society. 



very properly supports the teaching of the schools. If, says he, 

 an aberrant morphological type of organism appears in my cul- 

 tures, it is one of three things — it is either a contaminant, or it is 

 an involution form, or, if he is a brave man, it is a mutant. But 

 whichever of the three he thinks it is, he throws away his 

 cultures and starts afresh. And he does this because he is firmly 

 convinced that the lower bacteria can only reproduce themselves 

 — apart from endosporulation of a special type in certain cases — 

 by equal binary fission, this belief being itself founded on the 

 unquestioned fact that bacteria do breed true to type in stand- 

 ardized laboratory media. 



But, as you may remember, I ventured in answer to these 

 arguments to formulate four propositions : — 



1. That it is just as dangerous to assume that a given organism 

 is a contaminant, without the most searching enquiry, as it is to 

 assume that it is not. 



2. That to call an aberrant morphological type an involu- 

 tion form, especially when it occurs in young and suitable media, 

 is to take refuge behind a label which under such circumstances 

 means nothing at all. ' 



3. That the fact that bacteria breed true to type (to labora- 

 tory type) in test-tubes under the highly artificial condition of 

 laboratory culture cannot reasonably be taken as evidence that the 

 test-tube method of reproduction is the sole method of reproduc- 

 tion under natural conditions. 



4. That if the orthodox view as to the simplicity of the life- 

 cycle of bacteria be true, the phenomenon has practically no 

 parallel in comparative biology. 



On theoretical grounds alone, therefore, I urged that the present 

 view is not likely to be the correct view, and that in order to 

 examine the evidence for the opposite view with an impartial mind, 

 it is absolutely essential that we should not invoke too easily the 

 contaminant, the involution form, the mutant, et lioc genus omne, 

 in order to escape our difficulties. 



This, of course, is not to say tliat tlie most rigid precautions 

 against error should not be taken, as the description of the technique 

 to be presently describe! will, I hope, show. 



It is, of course, fully recognized that innumerable observations 

 have been recorded in the past of aberrant morphological types 

 occurring in apparently pure cultures of bacteria, especially when 

 their cultural environment was altered. Excellent illustrations 

 of this may be seen, for example, in Besson's pictorial description of 

 the Bacillus pyocyaneus, and of the B. antliracis. The first point, 

 however, 1 wish to make is, that if patience be employed it is often 

 possible to show the presence of aberrant morphological types 

 without change of cultural environment. It is therefore un- 

 necessary, provided that optimum conditions of cultivation are 



