MATERIALS OF EVOLUTION 43 



tatioii that evolution may still be taking place, and 

 that it may not be so slow as to be beyond our powers 

 of observation and even experimentation. The study 

 of living things has, I think, confirmed this expecta- 

 tion despite the fact that there exist at present rather 

 wide differences of opinion as to the legitimate con- 

 clusions from the evidence. 



A great deal of the discussion about evolution has 

 centered about the Ongin of Species. Historically, 

 the question as to what constitutes a species goes 

 back at least as far as Linne (1707-1778) who clas- 

 sified all plants and animals known to him into such 

 groups. The systematic arrangement of living things 

 into species, genera, families, etc., still attracts the 

 attention of a large number of naturalists. 



JNIany difficulties arise Avhen an attempt is made 

 to arrange animals into groups. It is generally rec- 

 ognized that, in some groups, species have a different 

 value from that in others. While some taxonomists 

 prefer to arrange individuals into large species, 

 other systematists split these large species into sev- 

 eral or many smaller ones, still calling them species. 

 It is generally admitted that the classification into 

 species is often an arbitrary procedure, one that is 

 useful, of course, in order to indicate the resem- 

 blances and differences that a study of animals and 

 plants reveals. A few students of the subject still 

 attempt to arrange their species in such a way as to 

 indicate their relationship by descent, but the at- 



