612 
Proceedings of the Royal Society 
the theory of pleomorphism. Crookshank ( Bacteriology , 1886), who 
adopts Zopf’s classification, says that in classifying species of bacteria, 
we must take into account — 1. Microscopical appearances in various 
nutrient media. 2. Character of their colonies under a low power, in 
plate cultivations. 3. Microscopical appearances of the organisms 
themselves. 4. Their physiological action. These desiderata, how- 
ever, scarcely satisfy the requirements of the case, and I beg leave to 
add, 5. The study of their modes of development and reproduction 
in each species. 
In this way w T e may be able to make out the life-history of an 
organism. Recent observations by Lister, JSTeelsen, Zopf, Van 
Tieghem, Klein, and Hauser, are said (Crookshank) to show that the 
orders of Cohn pass into one another. It is due to Lister to say, how- 
ever, that he retracted his first opinion with regard to the bacillus 
of black milk, and expressed a doubt as to whether he had not got a 
mixture of organisms. If this were true, of course, Cohn’s classifi- 
cation of genera would fall to the ground. Cohn’s strong point is 
that he opposes change of shape in genera. He admits differences 
in size and arrangement and colour, as well as physiological action, 
in the different species of each genus. The truth appears to be, that 
while the various genera retain their globular, oval, rod-shaped or 
spiral form throughout life, the different species described as cocci, 
rods, threads, and spirals may be merely stages of growth of a single 
organism. At the same time these various forms may produce 
different physiological effects. We find so-called “ cocci ” described 
as growing into torulse or rods. But we find that rods are spore- 
bearing, so that it appears rather a misnomer to call such spores 
cocci. Should they not be called spores of bacilli ? It is evident 
that a gap has to be filled up between the higher torula-form and the 
spore. The researches of Dr Douglas Cunningham upon the micro- 
organisms found in the intestinal canal show that they have an 
active and a resting stage. In suitable media, the “zoospores” 
multiply till the material for their growth is exhausted, when the 
medium is acid, and the micro-organisms cannot develop further. 
When transferred to an alkaline medium they at once become active. 
The mode of reproduction of bacterium termo has been well 
described by the Bev. W. H. Dallinger, in a remarkable paper 
communicated to the Royal Society of London in 1878 by Professor 
