42 John Henry Covistock 



natural sequence of groups should be represented by con- 

 stantly branching lines rather than by a single straight line. 



It seems to me that the most practicable way of meeting this 

 difficulty is to begin with the description of the most general- 

 ized form known, and to follow this with descriptions of forms 

 representing a single line of development, passing successively 

 to more and more specialized forms included in this line. 

 When the treatment of one line of development has been com- 

 pleted take up another line beginning with the most general- 

 ized member of that line and clearly indicating in the text that 

 a new start has been made. 



Much aid can also be given by a tabular statement of the 

 essential characters of the subordinate groups, using the form 

 of the ordinary analytical table. An illustration of this is 

 given in Part III of this essay. 



In this connection reference should be made to the proper 

 position of degraded forms in a series where an effort is made 

 to represent the natural sequence. The common practice of 

 assigning such forms the same position that would be assigned 

 to them if their simplicity of structure was the result of a, 

 primitive condition seems to me illogical. An example will 

 make this point clear. The Hemiptera are doubtless all de- 

 scended from a common winged ancestor. The lice, although 

 more simple in structure than most other members of the 

 order, do not represent the form of this ancestor as closely as 

 do the winged members of this order. They should not, 

 therefore, be placed first in the hemipterous series as is com- 

 monly done. It would represent the facts of nature better to 

 place them last, as forms departing more widely from the 

 primitive type of the order than do the winged forms. But it 

 should be clearly indicated that although they represent the 

 tip of one of the lines of development that line is a downward 

 bending line. 



In attempting to work out the phylogeny of a group of 

 organisms, there will arise, I believe, the necessity of distin- 

 guishing between two kinds of characters : first, characters 

 indicating differences in kind of specialization ; and second, 

 characters indicating differences in degree of specialization of 



