44 EVOLUTION AND GENETICS 



tempt, while desirable from the standpoint of evo- 

 lution, has proven so difficult that it has been tacitly 

 abandoned or ignored by many taxonomists. 



It is unfortunate, in my opinion, that ever since 

 the time of Darwin the question of the origin of spe- 

 cies has occupied so much of the forefront of specu- 

 lation concerning the evolution of living things ; for 

 if, as I have stated, "sj^ecies" are to a large extent 

 arbitrary arrangements of animals and f)lants — - 

 arrangements that may be essential for the purpose 

 for which they are made, but entirely unsuited for 

 evolutionary study — many unnecessary difficulties 

 may arise in an attempt to explain their origin by 

 natural processes, if some species are only groups 

 of individuals arranged by taxonomists for con- 

 venience. 



Nevertheless, there are certain important con- 

 siderations connected with attempts to classify liv- 

 ing things into species that cannot be dismissed by 

 the foregoing treatment of the subject. 



In the first place, experience has shown that most 

 animals and plants do fall into groups that are more 

 like each other than like other grouj^s; and in the 

 second place, that within each group the individuals 

 freely cross and leave fertile offspring, while be- 

 tween most species crossing is rare or impossible, 

 and when it occurs the offspring are generally ster- 

 ile. When two types are infertile with each other 

 they are generally admitted to be "good species" 



