LECTURE XXXV. 

 APPLICATION OF THEORY OF DESCENT TO TAXONOMY. 



Prof. John IT. Comstock. 



Definition of the terms generalized and specialized : 



Specialization by addition. Hind legs of grasshop- 

 pers. Maxilla3 of lepidoptera. Pectinations of an- 

 tennae. Tracheal gills. Wings of insects. 



Specialization by reduction. Loss of legs in ar- 

 thropoda. Loss of mandibles in lepidoptera. 



Modification of the form of parts through change 

 in function. The swim-bladder of fish a degenerate 

 lung. Dermal glands of insects changed into tracheae. 

 Tracheae furnished by addition with trachea! gills. 

 Tracheal gills changed into wings. If these supposi- 

 tions be true, the wings of insects were derived from 

 dermal glands. 



Myrick's law of lost organs : When an organ has 

 wholly disappeared in a genus other genera, which 

 originate as off-shoots from this genus, cannot regain 

 this organ, although they may develop a substitute 

 for it. Arthropods with more than six legs not derived 

 from the hexapoda. Insects with mouth part not 

 derived from mouthless insects. Winged insects not 

 derived from forms that have lost their wings. 



Rapidity with which organs are lost when they be- 

 come unnecessary. Wings of females of certain moths. 

 Hence a form lacking an organ may be only slightly 

 removed in other respects from the progenitor of forms 

 that still possess that organ. 



