Bacteria 27 



organisms, like the spirochaeta, appear sometimes in the group of 

 plants among the bacteria, and in other writings in the group of ani- 

 mals among the protozoa, is a matter of small consequence so long 

 as the knowledge of the organisms themselves is in no particular 

 diminished by the method of classifying them. 



In discussing the matter Delage says, "The question is not so 

 important as it appears. From one point of view and on purely 

 theoretic grounds it does not exist, while from another standpoint 

 it is insoluble. If one be asked to divide living things into two 

 distinct groups, of which one contains only animals and the other 

 only plants, the question is meaningless, for plants and animals are 

 concepts which have no objective reality, and in nature they are only 

 individuals. If in considering those forms which we regard as true 

 animals and plants we look for their phylogenetic history and decide 

 to place all of their allies in one or the other group, we are sure to 

 reach no result; such attempts have always been fruitless." 



"Huxley pointed out as early as 1876 the extremely close relation- 

 ship between the lowest algae and some of the flagellates, and it is 

 the general opinion that no one feature separates the lowest plants 

 from the lowest animals, and the difficulty — in many cases the 

 impossibility — of distinguishing between them is clearly recognized. 



"The point of view which demands a strict separation of animals 

 and plants has, however, little utility save, perhaps, to determine 

 the limits of a text-book or a monograph."* 



The now accepted relative position of the pathogenic vegetable 

 micro-organisms to the other vegetable organisms can be determined 

 by reference to the following table. The wide separation of the bac- 

 teria in Group II. and all of the others, which appear in Group X., 

 should be noted. 



The various genera to which the pathogenic fungi belong are by 

 no means closely related to one another, as can at once be seen by the 

 following amplification of Group X. Eumycetes: 



Size. — Bacteria are so minute that a special unit has been adopted 

 for their measurement. This is the micron, micromillimeter or 

 jit, and is the one-thousandth part of a millimeter, equivalent to the 

 one-twenty-five-thousandth (3^5ooo) of ^^ inch. 



There is no limit to the minuteness of micro-organisms. Visibility 

 is no longer a criterion. There are micro-organisms that can be 

 seen with low powers, others that can only be seen with high 

 powers, and a few that probably cannot be seen with any power of 

 the microscope. These are called "invisible viruses." They are 

 known to us through the biological quahty of filtrates in which they 

 are present because of their abihty to pass through the pores of the 

 filters.. For this reason they are also called "filterable viruses." 

 As they cannot be seen, we have no way of classifying them; they 

 may be bacteria or protozoa, or neither or both. 

 * Calkins', "The Protozoa," p. 23. 



