19 



bacteria growing under different circumstances, are mor- 

 phologically indistinguishable, one is not always certain 

 that the organisms found are really the offspring of those 

 planted rather than morphologically identical intruders. 

 Suppose, for example, we are attempting to isolate the 

 micrococci found in the blood of a pysemic patient ; on 

 examining a drop from our tenth generation, our tenth 

 successive flask, we find only micrococci identical 

 in appearance with those planted. Are we warranted 

 in asserting that these are the descendants of the 

 organisms seen in the blood ? To answer the question 

 affirmatively we must assume first that the micrococci 

 contained in the blood were the only ones which gained 

 access to the first flask ; then that our ten flasks, ten 

 liquids, ten stoppers were rendered and remained sterile ; 

 that our pipettes, forceps, etc., have been always free from 

 organisms, and that during the nine transfers, and perhaps 

 numerous test examinations, no micrococci have gained 

 access to our cultures from the air an assumption not 

 warranted by experience. By such assumptions Bastian 

 demonstrated spontaneous generation. 



A source of possible error in these methods, not al- 

 ways recognized, is the necessary assumption that ap- 

 parent community of form among bacteria proves identity 

 of function. The fallacy of such assumption is d priori 

 evident, and has been repeatedly demonstrated. A Cy- 

 clops whose exaggerated retina might fail to distinguish 

 objects less than three feet in diameter, could perceive 

 no morphological characteristics wherein a wolf differs 

 from a sheep, and could have no conception of the per- 

 versity which distinguishes a mosquito or a wasp from 

 the house-fly. In 1875 even Cohn pronounced the harm- 

 less bacillus subtilis of hay infusion morphologically iden- 

 tical with the bacillus anthracis ; to-day we can, thanks 

 to improvements in technique, distinguish the latter not 

 only from the hay bacillus, but also from two other impor- 

 tant varieties which are morphologically extremely similar, 

 though physiologically quite distinct. Further research 

 may possibly reveal differences of form even among the 



