It is impossible from any point of view, morphological, biological or serological, to draw a sharp dividing line in this series. The forms change gradually from the mycelial organism to the bacillary, acid-fast organism. It is biologically a group complex and should be so considered. No doubt experiments with a larger series of species would yield results giving a possibility of closer classification and the introduction of some of the forms now in a debatable position, as Bacillus diphtheriæ and Bacillus mallei, and other organisms, sometimes called, on account of their morphological irregularities, corynebacterium and mycobacterium, would help to show their real relation to both the Streptothrices and the true bacteria. The latter in many ways are acknowledged to be far from primitive; their endospores, flagella, and food habits all indicate a relatively high degree of specialization. Hence it would seem biologically more reasonable to look upon this group of Streptothrices with their variable morphology and close relationships as representing the ancestral type that gave rise to both the higher fungi and true bacteria, and not as being themselves higher bacteria. The various bacteria, other than the acid-fast forms, can readily have arisen from the non-acid-fast bacillary types, or even as non-acid-fast specializations of the mixed types. All the various forms shown at present by the bacteria,—cocci, spirilla, bacilli, etc.,—either separate or in chains and masses, are to be recognized in this group, and specializations in one or another line in the past would readily have given rise to the types we consider true bacteria. The processes of evolution have carried them far away from the parent stock and made them into this group. The recognition of this group complex and of the intermediate forms indicates clearly the past history and present relations of these interesting organisms. These relations may be represented by the following scheme.

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It is probable that the relation between the acid-fast organisms and the Streptothrices is a closer one than that between the Streptothrices and the bacteria, perhaps close enough to warrant a common genus for both.

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