THEIR PLACE IN NATURE. 35 



organic substances ; while the parasitic group exists 

 only at the expense of the more highly organized mem- 

 bers of both the animal and vegetable kingdoms. It is 

 to the parasitic group that the pathogenic 1 organisms 

 belong. 



In addition to the metatrophs that are concerned in 

 the changes to which allusion has just been made, there 

 exist other species whose life-processes result in specific 

 changes of great interest and importance. Some of 

 these are characterized by their property of producing 

 pigments of different color; these are known as the 

 chromogenic 2 species. Just what the biological signifi- 

 cance of the pigment-producing function is cannot 

 be said, but the fact that many of the chromogens 

 are richly endowed with proteolytic 3 activity makes it 

 probable that they are, in common with other meta- 

 trophs, concerned in the omnipresent process of disinte- 

 gration in progress in all dead organic matters, and 

 that, after all, their power to produce colors, though 

 conspicuous, is of but subordinate importance. From 

 the investigations of Beyerinck it would seem that in 

 some of the chromogenic forms the pigment is an 

 integral part of the bacteria themselves ; in others that 

 it is an excretory product of species that are them- 

 selves colorless ; while in still others, that it is an 

 excretory product which remains intimately associated 

 with the bacterial cells and in part or wholly stains 

 them. 



Others, the so-called photogenic or phosphorescent 



1 Pathogenic organisms are those which possess the property of pro- 

 ducing disease. 



2 Chromogenic: possessing the property of generating color. 

 3 Proteolytic : the power of dissolving or digesting proteids. 



