120 ILLINOIS BIOLOGICAL MONOGRAPHS [438 



IV. PHYLOGENY 



A classification based on phylogeny is one of the essential concerns of 

 philosophical taxonomy. In order to ascertain the genetic relationship 

 of organisms, synthetic as well as analytic, consideration of evidence 

 drawn from all the branches of biological science is imperative. The 

 indissoluble relation of morphology, embryology, and paleontology to tax- 

 onomy is so manifest and familiar that no comments are needed. The 

 time has come, however, when a critical examination of the phylogenetic 

 significance and the taxonomic value of the physiological and biological 

 attributes of animals must be made. Whatever evidence comparative 

 physiology, biochemistry, and genetics may offer should be incorporated 

 as far as possible with the data obtained in other more commonly ex- 

 ploited fields of research. Only in this way is it possible to arrive at a 

 comprehensive, systematic and complete summation of knowledge of 

 animals. This is the primary function of philosophical taxonomy, and in 

 this sense the saying of W. S. Jevons that "science can extend only so far 

 as the power of accurate classification extends," is true. 



There are good reasons to believe, however, that even to-day morphol- 

 ogy, as of old, holds its supreme place in systematic investigations as it 

 offers fundamental assistance in determining the genetic affinities of 

 organisms. The success of a study of phylogeny based on morphological 

 evidences depends on the ability of the investigator to select the proper 

 structures, to determine the direction and nature of changes undergone by 

 these structures, and to draw legitimate conclusions by judicious interpre- 

 tation of the facts observed. Data obtained from studies of the external 

 anatomy of the larval stages of entometabolous insects are of necessity 

 incomplete of themselves for determining the phylogeny of the group; 

 yet, in the absence of other means of approach to the problem, they 

 constitute essential facts significant enough to merit careful consideration. 



The opinions of scientists in regard to the systematic importance of 

 the characters based upon the immature stages of Entometabola have 

 been divided. There are some who ascribe no importance whatsoever to 

 them and entirely ignore this phase of taxonomy. There are others who 

 recognize the importance of the larvae from the viewpoint of synoptic 

 classification as they are primarily interested in the practical purpose of 



