562 W. L. TOWER, 



acquired characters, such as the lateral projection from the sides of 

 the thorax of the primitive pterygote insects, are ever inherited and 

 the discussion of the perfection and modification of this as given by 

 Packard (1898) is pure speculation, and v^hile use, inheritance, selection 

 and the inheritance of acquired characters may and probably have 

 all played important parts in the genesis of insect wings, there is as 

 yet no evidence to show what part or parts have been taken by these 

 factors even in the later development of insect structures. No evidence 

 has yet been but forward by those who support this theory which 

 gives even a remote clue as to the origin and homology of insect wings. 



That the wings are derived from structures like tracheal gills 

 has been advocated by Gegenbaur and others and is equally wanting 

 in convincing evidence to support it. Thus, according to Gegenbaur, 

 "The wings of insects must be regarded as homologous with the 

 tracheal gills, for not only do they agree with tbem in origin, but 

 also in their connection with the body and in structure. Their being 

 limited to the second and third thoracic segments points to a reduction 

 in the number of tracheal gills. It is clear that we must suppose the 

 wings did not arise as such but were developed from organs which 

 had another function, such as tracheal gills; I mean to say such a 

 supposition is necessary, for we cannot imagine that the wings functioned 

 as wings in the lower stages of their development, and that they could 

 have been developed by having such a function." 



To this hypotheses two very strong objections at once arise, 

 first: It presupposes that the ancestors of the Pterygote insects were 

 aquatic forms having tracheal gills ; but the major part of the evi- 

 dence as to the ancestry of Pterygota points directly to a terrestrial 

 form, perhaps not unlike Campodeu; second, tracheal gills, as Packard 

 (189^) clearly points out, are secondary, adaptative and temporary 

 larval structures and are not of phylogenetic significance. Moreover 

 the fundamental type of wing tracheation so clearly brought out 

 by CoMSTOCK & Needham (1899) is not the same as the trache- 

 ation of any tracheal gill that I have been able to discover, and it 

 is reasonable to suppose that if wings are derived from tracheal gills 

 there should be a fundamental resemblance between the tracheation 

 of the two. 



The connection of the wings with the spiracles, as shown by 

 Verson (1890) and in this paper, does not help the tracheal gill theory, 

 because the spiracles are not developed from or in connection with 

 tracheal gills. Müller (1875) states positively that the "wings of 



