2 



which agreed closely with that form in the absence of all 

 visible structure and differentiation. From such an ancestor 

 the derivation of the various Monera now living, and of other 

 allied organisms which have probably existed at different 

 periods, can be readily imagined to have taken place by 

 slight changes of form, habit, and life-history, effected by 

 means of natural selection. It is quite possible that some 

 existing or extinct groups of lowest organisms may have 

 arisen independently of others, but there is no evidence in 

 favour of this " polyphyletic " arrangement, and therefore it is 

 simpler to understand, and on the whole more probable, that 

 all forms of life have been derived from a single common 

 ancestor. When once a particle of living protoplasm had, 

 under certain conditions of which we are absolutely ignorant, 

 become evolved from inorganic materials, there would be no 

 need of any further points of origin. The various Protista 

 (the lowest animals and plants) can all be satisfactorily 

 regarded as being derived from one another, or from hypo- 

 thetical intermediate forms. 



The line leading straight * upwards from Protamoeha to 

 near Protomyxa is supposed to run through a series of ances- 

 tral Monera which were successively less and less absolutely 

 undifferentiated, and from which many side branches (long 

 or short, according to the amount of variation displayed) 

 have diverged. The few of these which are shown in the 

 table may be taken as representing some of the more im- 

 portant groups of Monera which are known. The inter- 

 mediate forms have become extinct.! The table would be a 

 more correct representation of nature if every line and branch 

 had been shown bristling with innumerable short twigs, 

 extending in all directions, of different lengths, and many of 



* All the straight lines in the table must be regarded as very fine zig- 

 zags, since each ancestral form diverged a little from its predecessor. 

 t In regard to this, consult The Origin of Species, 6ih ed., p. 293. 



