124 ELEMENTS OF BIOLOGY 



be placed in the sub-family Muscinae. Its mouth parts are not adapted 

 for piercing and in other minor characters it agrees with descrip- 

 tions of the genus Musca. Its size, method of breeding, minor char- 

 acters of its wings and other structures enable us to regard it as 

 identical in form with a fly which is quite common and the species 

 name domestica is assigned to it. Recapitulating, the unknown has 

 now, in our minds at least, been placed in its proper relation with 

 all other living forms: 



Kingdom — Animal 

 Sub-kingdom — Metazoa 

 Phylum — Arthropoda 

 Class — Insecta 

 Order — Diptera 

 Family — Muscidae 

 Sub-family — Muscinae 

 Genus — Musca 

 Species — domestica 



Common name — house fly 



Phylogenetic Relationships. The order of probable actual 

 relations between the different phyla is conventionally represented 

 by a diagram (Fig. 75) variously termed "the tree of life," "the 

 phylogenetic tree" or other terms indicating probable descent of 

 the more complex animals from the simpler forms. Too much 

 confidence must not be placed in the accuracy of this diagram, for 

 the relations represented are in many cases still matters of dispute. 

 Hence other diagrams have also been arranged that satisfy an order 

 in relations. Moreover, it is extremely probable that no phylum that 

 we recognize among present-day animals is descended from any 

 other modern forms. The diagram assists in visualizing in a broad 

 way a scale of complexity and the general relations between pres- 

 ent-day types. The position of an animal with reference to other 

 phyla is commonly termed its phylogenetic relationship. The term 



