No. 564] 



THE FIXATION OF CHARACTER 



725 



establishment of much more precise taxonomic criteria, in 

 a clearing up of many difficulties and disputes as to rela- 

 tionships and in the construction of a truly natural classi- 

 fication on a more logical and consistent basis. 



A knowledge of phylogenetic principles is also of value 

 to the general student of evolution, for through it a better- 

 conception of the development of organic structures may 

 be obtained than is set forth by the selection theory. A 

 recognition of the facts that fixity increases with differ- 

 entiation and that there are inherent differences in vari- 

 ability between functionally important characters and 

 those which are useless for survival makes possible a 

 much clearer understanding of the evolutionary history 

 of any particular group. 



The evolution of the hexapod insects is a case in point. 

 The primitive insects seem to have been air-breathing 

 arthropods with an indeterminate number of body seg- 

 ments and appendages. The ancestors of our modern 

 hexapods achieved their first success through some ad- 

 vance in specialization over this more primitive type, 

 but the improvements which gave them ascendancy and 

 which enabled them to found a distinct and dominant 

 group were certain unknown changes, doubtless in plastic 

 and functionally important characters which were of great 

 value for survival at the time, but which, having isolated 

 the family and put it on its feet, so to speak, continued 

 to change and may be possessed by few or no living de- 

 scendants. The progressive increase in specialization, 

 however, which caused the success of the primitive hexa- 

 pods resulted in the gradual fixation of certain function- 

 less characters, such as the number of segments and ap- 

 pendages, which finally became rigidly stereotyped as we 

 see them to-day, so that they now distinguish all hexapods. 

 whether successful and dominant species or those which 

 are being beaten and exterminated. The conservative 

 features have progressed steadily but slowly to then- 

 present condition, but the plastic characters, during the 

 same time, have doubtless passed through wide and unre- 



