454 



III. — The stem must be determined. Before an agreement can be 

 reached as to the phylogenetic status of any form, there must be 

 agreement as to what constitutes specialization, and what generalized 

 conditions. Suppose, for example, that within an order of insects 

 there are species with two types of wings — one having numerous 

 cross-veins and the other but few; which is the more specialized? It 

 is possible for either type to have been derived from the other or 

 both to have arisen from a third extinct form. One may have be- 

 come specialized "by addition" and the other "by reduction". In this 

 case it is evident that the stem must first be determined beTore the 

 degree of specialization of either form can be stated with accuracy. 



IV. — All possible characters should be taken into account, and a 

 decision concerning the rank of the group should be based on a study 

 of the whole organism. This method should be followed in view of 

 the fact that the same degree of specialization in structure is not 

 usually found simultaneously in different parts of the body, and it is 

 always to be preferred to the method of determining specialization 

 or generalization of a group of organisms by the study of a few 

 characters. 



V. — The forces which produce modification in structure should 

 be recognised if possible and their effect upon structure determined. 



In the following comparisons the various characters will be con- 

 sidered separately and, where possible, the stem form will be men- 

 tioned and the reasons given for so regarding it. For convenience, 

 the division of the suborders into families as outlined by Handlirsch 

 ('o6-'o8) and Muttkowski ('10) will be followed, the Zygoptera be- 

 ing divided into the Agrionidae and Coenagrionidae ; the Anisoptera, 

 into the Aeshnidae, Gomphidae, and Libellulidae. 



Bgg 



i. — Eggs of the Odonata are of two types; one long and some- 

 what cylindrical in shape, the other ellipsoidal and short. The dif- 

 ferences in shape are the result of different methods of oviposition. 

 The ellipsoidal form would seem to be the more primitive, judging 

 from a general knowledge of the eggs of various orders of insects. 

 No definite proof of this can be given, but a comparison with the 

 eggs of the Apterygota and the lower Arthropoda indicates that the 

 ellipsoid is probably the stem type. This is, however, in direct con- 

 tradiction to the argument found in the reduction of the ovipositor, 

 since the species with specialized or reduced ovipositors lay ellipsoidal 

 eggs. Disregarding the latter argument and considering the ellipsoi- 



