56 Causes and Course of Organic Evolution 



The iron, the sulphur, and the purple bacteria seem all to 

 be able to utilize inorganic substances for their nutrition and 

 for supplies of energy. But a physiological group of immense 

 importance and widespread distribution at the present day 

 is that of the saprophytic species. These utilize decaying 

 products of other organisms for their support, and so probably 

 branched off early from some one of the above autotrophic 

 lines, as plants and animals became increasingly abundant and 

 in time underwent decay. 



A fifth group, derived almost certainly from some one of 

 the above four, attracted chemotactically by the rich food and 

 body tissues of these, gradually assumed a parasitic mode of 

 life. The numerous species of bacterial parasites now living 

 is proof that in past stages of organic evolution they may have 

 been at least equally abundant, and doubtless then, as now, 

 were responsible for widespread destruction of higher plants 

 and animals. Later we shall adduce evidence that favors the 

 origin of primitive animal forms from either the saprophytic 

 or parasitic bacteria. 



While many of the species or even genera like Sarcina are 

 inert, passive, and permanently embedded in mucilaginous 

 wall substance, graded transitions can be traced to types that 

 are highly active and responsive physiologically. It is also 

 amongst the latter forms that a cell nucleus, or its fairly formed 

 rudiment, has been asserted to exist. 



Irritable or chemotactic response to the presence or absence 

 of oxygen, carbon dioxide, and many other chemicals is sim- 

 ilarly graded in its manifestations. Equally true is this for 

 thermic, lumic, and electric stimulation, as the works of Engel- 

 mann (38), M. Ward, Jennings (42), and others demonstrate. 

 It need scarcely be added then that we know much more re- 

 garding them than the Schizophycese as to physiological re- 

 sponse. This, however, is in many respects a matter for regret, 

 as the latter if sufficiently studied would give us in all likelihood 

 valuable and fundamental results. 



In geographical distribution the bacteria excel again, for 

 it may well be said that they are omnipresent and abundant, 



