It is obvious that the wider the distribution the wider is the area of tissue change, and _vice versâ_. Yet there is something of far greater importance than the mere presence of bacteria in human or animal tissues; for the secondary action of disease-producing germs--and possibly it is present in all bacteria--is due to their poisonous products, or _toxins_, as they have been termed. These may be of the nature of ferments, and they become diffused throughout the body, whether the bacteria themselves occur locally or generally. They may bring about very slight and even imperceptible changes during the course of the disease, or they may kill the patient in a few hours. Latterly bacteriologists have come to understand that it is not so much the presence of organisms which is injurious to man and other animals, as it is their products which cause the mischief; and the amount of toxic product bears no known proportion to the degree of invasion by the bacteria. The various and widely differing modes of action in bacteria are therefore dependent upon these three elements: the tissues or medium, the bacteria, and the products of the bacteria; and in all organismal processes these three elements act and react upon each other. A word may be said here respecting the much-discussed question of _species_ in bacteria. A species may be defined as "a group of individuals which, however many characters they share with other individuals, agree in presenting one or more characters of a peculiar and hereditary kind with some certain degree of distinctness."[9] Now, as regards bacteria, there is no doubt that separate species occur and tend to remain as separate species. It is true, there are many variations, due in large measure to the medium in which the organisms are growing,--variations of age, adaptation, nutrition, etc.,--yet the different species tend to remain distinct. Involution forms occur frequently, and degeneration invariably modifies the normal appearance. But because of the occurrence of these morphological and even pathological differences it must not be argued that the demarcation of species is wholly arbitrary. _Means of Sterilisation._ As this term occurs frequently in even a book of this untechnical nature, and as it is expressive of an idea which must always be present to the mind of the bacteriologist, it may be desirable to make some passing allusion to it.