THE KVOLUTIONARY R 1,1, A T I () N S H I l>S OV BACTI-RIA I53 



The flagellation of typical Bactcriaccac represents an intermediate sta^e of 

 evolution in this respect. 



If any weight whatsoever can be placed upon the theory of recapitulation, 

 the fact that the flagella in germinating microcysts of Bactcriaccac appear first 

 in the polar position must be regarded as of some interest. 



The problem of movement on land has been solved in a different manner 

 by myxobacteria, which have adapted the cell wall to a crawling action, as in 

 the Myxophyccae. The evidence at present available does not enable it to 

 be determined whether this character is indicative of a relationship with the 

 Myxophyceac or, through the more flexible spirilla and trichobacteria, with 

 the typical bacteria, but cytological evidence is strongly in favour of the latter. 



E: AERIAL DISTRIBUTION 



The most highly evolved bacteria appear to be those which have not only 

 colonised the terrestrial habitat successfully, but have also developed adaptations 

 for the aerial distribution of their reproductive elements. Of these the best 

 example is provided by the streptomyces, which (presumably by evolutionary 

 convergence) resemble minute moulds with aerial conidia, although their 

 cytological structure and behaviour reveals their affinities with other bacteria. 

 Here also, a complete series of forms of varying degrees of complexity serves 

 to connect them, through Nocardia TmA Mycobacteria, with the Gram-positive 

 eubacteria, among which other devices for aerial distribution may be found. 



Consideration of this tendency provides a possible explanation ot the 

 nature oi the bacterial endospore. Although less efficient in this respect than 

 those of the streptomyces, the spores of Gram-positive bacilli are capable of 

 being distributed in air and dust, and it is worthy of consideration whether 

 it may be this factor, rather than their remarkable powers of resistance, which 

 has conferred a genetical advantage upon their possessors. 



Although the exact differences between the physico-chemical constitution 

 of spores and vegetative cells of the same species remain obscure, it has 

 repeatedly been claimed that the water-content oi the former is reduced. 



