636 The American Naturalist. [August, 
D. Discovery of germs in the inoculated, diseased tissues, re- 
isolation of the same, and growth on various media until it is 
determined beyond doubt that they are identical with the 
organism which was inoculated." 
Not one of these steps can be omitted. Possible sources of 
error beset the investigator at every step, and anything short 
of a rigid demonstration cannot be accepted as proof. A. is 
usually quite easy, involving only the careful microscopic ex- 
amination of abundant material, stained and unstained. B. 
was made possible by the improvement of methods, i. e. by 
the use of solid media, and especially by the discovery of the 
method of isolation by means of plate cultures. C. is quite 
easy, provided the right organism has been obtained and 
this be inserted into the proper tissues under the right con- 
ditions to insure growth. The fulfillment, however, of these 
conditions often involves long and vexatious delays, and taxes 
the acumen of the investigator to the utmost. D. serves as a 
check on all the preceding work, showing that there has been 
no unintentional mixing of organisms, and that the results 
obtained were actually due to the supposed cause. For the sake 
of brevity these four rules of practice will be referred to in the 
following pages simply as A. B.C. and D. What weight shall 
be given any specific statement depends of course on the reputa- 
tion of the individual. Some men are so careful of their 
reputation and so little given to making unwarranted state- 
ments that their simple word goes a long way even when the 
statements themselves seem improbable, whereas the elaborate 
explanations of other men, if the asserted facts are at all out 
of the ordinary, have to be taken with a grain of salt. 
The requirements under the second head, i. e. Description of 
the organism, are more numerous, and are embraced under two 
general divisions of very unequal value, namely Morphology 
and Biology. In the classification of the higher plants and 
animals morphology has been accepted from time immemorial 
14 A series of successful Armer is always very desirable and becomes in- 
dispensable in case the infecti re obtained on plants grown in a locality where 
the disease prevails sanai "Of course, numerous un-inoculated plants, known 
as “checks ” or “ controls,” must always be reserved for comparison. 
