THE FIRST LIVING THINGS 



217 



appeared on the earth. In the first place we may safely assume 

 that they were extremely simple in structure, for it is generally 

 agreed that the evolution of the higher forms of life has been 

 accompanied by a gradually increasing complexity of organization. 

 It is also certain that they cannot have been animals, for animals, 

 as we have seen in a previous chapter, are dependent for 

 their food supply upon other living things, being themselves 

 unable to build up the proteid molecule from inorganic constitu- 

 ents. Green plants are the great proteid manufacturers at the 

 present day, but only by virtue of the fact that they contain 

 chlorophyll, which enables them to utilize the energy of the sun's 



r w. 



I '4 



9 



r 



FIG. 83. Different Forms of Disease-producing Bacteria, X about 1500. 

 (From Strasburger, after Fischer.) 



a, pus cocci ; 6, erysipelas cocci ; c, gonorrhoea cocci ; d, bacilli of splenic fever; e, bacilli 

 of tetanus ; /, bacilli of diphtheria ; g, tubercle bacilli ; h, typhoid bacilli ; i, colon 

 bacilli ; k, cholera bacilli. 



rays in the process of photosynthesis. Now chlorophyll itself is a 

 very complex substance, and we cannot suppose that the first living 

 things already possessed it. If, then, they had no other organisms 

 to feed upon, and if they* possessed no chlorophyll, how did they 

 obtain the energy necessary to enable them to maintain life 

 at all? 



The simplest and at the same time the smallest organisms known 

 to us at the present day are the Bacteria (Fig. 83), many of which 

 are so minute as to be hardly visible even under the highest 

 powers of the microscope. These Bacteria are neither plants 

 nor animals, but occupy a position lower than either. They have 

 not even attained to the dignity of perfect cells, for they exhibit 

 no proper differentiation into cell body and nucleus, the chromatin 

 substance, which in a typical cell is aggregated in the nucleus, 



