638 THE BEGINNINGS OF LIFE. 



Infusorial and Cryptogamic Organisms which we have 

 classed under the name ^ ephemeromorphs.' Natural 

 Selection as a producer of variation is, indeed, wholly 

 limited to ^ species/ such as we now define them. 



But amongst specific forms of slight complexity, the 

 influence of Natural Selection as a modifying agent is 

 probably much less important than it is amongst more 

 active and complex animal forms; and in all cases its 

 action in producing change may be assisted by ' spon- 

 taneous' internal changes in the molecular activity of 

 certain parts of the organism, or by other internal 

 changes which are more definitely induced by modi- 

 fications in the sum-total of ' external conditions ' 

 acting upon the organism. 



Each cause of specific modification, however, whether 

 acting alone or in concert with one of the other pro- 

 ducers of internal change, can only come into play in 

 subordination to the ever-potent laws of ^organic 

 polarity,' by which a multiplication of effects is apt to' 

 be induced. 



An elemental origin of ^ living ' matter similar to 

 that which takes place at the present day, and in 

 addition all the related heterogenetic phenomena, have 

 probably been taking place on the surface of our globe 

 since the far -remote period when such matter was 

 first engendered. 



The countless myriads of livifig units which have 

 been evolved in different ages of the world's history 



