Even when used in conjunction with purely morphological characters, these physiological properties are too variable to aid us in the discrimination of species and genera, and are apt to break down at critical periods. Among the more characteristic of these schemes adopted at various times may be mentioned those of Miquel (1891), Eisenberg (1891), and Lehmann and Neumann (1897). Although much progress has been made in determining the value and constancy of morphological characters, we are still in need of a sufficiently comprehensive and easily applied scheme of classification, partly owing to the existence in the literature of imperfectly described forms the life-history of which is not yet known, or the microscopic characters of which have not been examined with sufficient accuracy and thoroughness. Fischer's Scheme. The principal attempts at morphological classifications recently brought forward are those of de Toni and Trevisan (1889), Fischer (1897) and Migula (1897). Of these systems, which alone are available in any practical scheme of classification, the two most important and most modern are those of Fischer and Migula. The extended investigations of the former on the number and distribution of cilia (see fig. 1) led him to propose a scheme of classification based on these and other morphological characters, and differing essentially from any preceding one.

This scheme may be tabulated as follows: -

I. Order - Haplobacterinae. Vegetative body unicellular; spheroidal, cylindrical or spirally twisted; isolated or connected in filamentous or other growth series.

1. Family - Coccaceae. Vegetative cells spheroidal.

(a) Sub-family - Allococcaceae. Division in all or any planes, colonies indefinite in shape and size, of cells in short chains, irregular clumps, pairs or isolated: - Micrococcus (Cohn), cells non-motile; Planococcus (Migula), cells motile.

(b) Sub-family - Homococcaceae. Division planes regular and definite: - Sarcina (Goods.), cells non-motile; growth and division in three successive planes at right angles, resulting in packet-like groups; Planosarcina (Migula), as before, but motile; Pediococcus (Lindner), division planes at right angles in two successive planes, and cells in tablets of four or more; Streptococcus (Billr.), divisions in one plane only, resulting in chains of cells.

2. Family - Bacillaceae. Vegetative cells cylindric (rodlets), ellipsoid or ovoid, and straight. Division planes always perpendicular to the long axis.

(a) Sub-family - Bacilleae. Sporogenous rodlets cylindric, not altered in shape: - Bacillus (Cohn), non-motile; Bactrinium (Fischer), motile, with one polar flagellum (monotrichous); Bactrillum (Fischer), motile, with a terminal tuft of cilia (lophotrichous); Bactridium (Fischer), motile, with cilia all over the surface (peritrichous).

(b) Sub-family - Clostridieae. Sporogenous rodlets, spindle-shaped: - Clostridium (Prazm.), motile (peritrichous).

(c) Sub-family - Plectridieae. Sporogenous rodlets, drumstick-shaped: - Plectridium (Fischer), motile (peritrichous).

3. Family - Spirillaceae. Vegetative cells, cylindric but curved more or less spirally. Divisions perpendicular to the long axis: - Vibrio (Müller-Löffler), comma-shaped, motile, monotrichous; Spirillum (Ehrenb.), more strongly curved in open spirals, motile, lophotrichous; Spirochaete (Ehrenb.), spirally coiled in numerous close turns, motile, but apparently owing to flexile movements, as no cilia are found.

II. Order - Trichobacterinae. Vegetative body of branched or unbranched cell-filaments, the segments of which separate as swarm-cells (Gonidia).

1. Family - Trichobacteriaceae. Characters those of the Order.

(a) Filaments rigid, non-motile, sheathed: - Crenothrix (Cohn), filaments unbranched and devoid of sulphur particles; Thiothrix (Winogr.), as before, but with sulphur particles; Cladothrix (Cohn), filaments branched in a pseudo-dichotomous manner.

(b) Filaments showing slow pendulous and creeping movements, and with no distinct sheath: - Beggiatoa (Trev.), with sulphur particles.

The principal objections to this system are the following: - (1) The extraordinary difficulty in obtaining satisfactory preparations showing the cilia, and the discovery that these motile organs are not formed on all substrata, or are only developed during short periods of activity while the organism is young and vigorous, render this character almost nugatory. For instance, B. megatherium and B. subtilis pass in a few hours after commencement of growth from a motile stage with peritrichous cilia, into one of filamentous growth preceded by casting of the cilia. (2) By far the majority of the described species (over 1000) fall into the three genera - Micrococcus (about 400), Bacillus (about 200) and Bactridium (about 150), so that only a quarter or so of the forms are selected out by the other genera. (3) The monotrichous and lophotrichous conditions are by no means constant even in the motile stage; thus Pseudomonas rosea (Mig.) may have 1, 2 or 3 cilia at either end, and would be distributed by Fischer's classification between Bactrinium and Bactrillum, according to which state was observed.

In Migula's scheme the attempt is made to avoid some of these difficulties, but others are introduced by his otherwise clever devices for dealing with these puzzling little organisms.

The question, What is an individual? has given rise to much difficulty, and around it many of the speculations regarding pleomorphism have centred without useful result. If a tree fall apart into its constituent cells periodically we should have the same difficulty on a larger and more complex scale. The fact that every bacterial cell in a species in most cases appears equally capable of performing all the physiological functions of the species has led most authorities, however, to regard it as the individual - a view which cannot be consistent in those cases where a simple or branched filamentous series exhibits differences between free apex and fixed base and so forth. It may be doubted whether the discussion is profitable, though it appears necessary in some cases - e.g. concerning pleomorphy - to adopt some definition of individual.

Fig. 12. Myxobacteriaceae. Fig. 12.

A. Myxococcus digelatus, bright red fructification occurring on dung.

B. Polyangium primigenum, red fructification on dog's dung.

C. Chondromyces apiculatus, orange fructification on antelope's dung.

D. Young fructification.

E. Single cyst germinating.

(A, B, after Quehl; C-E, after Thaxter.) From Strasburger's Lehrbuch der Botanik, by permission of Gustav Fischer.