220 



SCIENCE 



[N. S. Vol. XXV. No. 632 



nor was Dr. ^rauer; and, doubtless, many 

 another in days to come, observing the indi- 

 viduality anQ persistence of the trachese, and 

 the regular formation of the veins about them, 

 will be unable to ignore them with the serenity 

 of the author. 



For, in primitive insects, tracheae develop 

 first, and the veins later develop about them. 

 That is not theory, but fact; any one may 

 easily see it for himself. The very presence 

 of the trachese between the two membranes of 

 the wing when these are fusing sufficiently 

 accounts for the primary location of the veins. 

 Moreover, these trachese in generalized wings 

 show all the usual signs of homology: likeness 

 in form, likeness in relations and greater like- 

 ness in earlier than in later stages. If these 

 are not good evidence, there are no homologies 

 of any significance. Furthermore, the homol- 

 ogies discovered in the trachese are fully cor- 

 roborated by those of the adult veins, previ- 

 ously ascertained. 



The author concludes his argument for the 

 exclusion of the trachese as follows : " A large 

 amount of very strong evidence would be 

 needed to explain away the essential identity 

 of structure in tracheated and non-traeheated 

 veins; the evidence obtainable seems to indi- 

 cate identity rather than difference. We must 

 conclude, then, that the presence or absence of 

 a trachea is an incident of structure of no 

 special significance in comparing veins " (p. 

 47). Precisely. Just as the presence or ab- 

 sence of teeth in mammalian jaw bones is of 

 no special importance in identifying the bones. 

 Not the presence or absence of either teeth or 

 trachese is of chief significance, but the form 

 taken on when present. 



The precedence of the trachese and the sub- 

 sequent development of the veins about them 

 is disposed of in this way : " It is impossible 

 to deny that the location of the veins may 

 have been really marked out, though unrecog- 

 nizable to the eye" (p. 46). This position is, 

 of course, unassailable. Those who can attain 

 to equal faith may find equal security in it. 



If a ' rational theory of venation ' requires 

 the elimination of the trachese in order to get 

 room to grow in, let us take leave of them, as 

 does the author, and then see how the theory 



thrives. Without attempting to follow in de- 

 tail the hypothetical explanation of the man- 

 ner in which veins arise, through the activity 

 of a hypothetical substance secreted by the 

 hypodermis, forming folds or wrinkles, con- 

 trolled by hypothetical pressures, we find the 

 veins at length appearing after the following 

 hypothetical fashion: first a marginal vein 

 about the entire border of the wing, and a 

 pritnary vein along the middle of it in posi- 

 tion somewhat like that of the ridge on the 

 under surface of the gill cover figured. Then 

 there appear anterior and posterior veins in 

 the spaces at either side of the primary, out- 

 growing from the base of the wing. Finally, 

 a series of independent veins is formed by 

 ingrowth from the marginal vein toward the 

 base of the wing; and with subsequent attach- 

 ments and adjustments of these, the venation 

 is completed. 



In this theory of ingrowing independent 

 veins (branches of the median vein) ' a class 

 of veins that never had basal connections,' 

 lies the chief novelty of the paper. It becomes 

 at once evident why the trachese have had to be 

 excluded; for the branches of the median 

 trachea are not independent, and they grow 

 outward from the base of the wing like the 

 other trachese. It is, indeed, surprising that 

 greater care has not been taken to establish 

 an hypothesis intended for general application 

 on a better basis of facts. The only evidence 

 given to show that these veins are really in- 

 growing is that they are usually weaker to- 

 ward their inner ends and are sometimes in- 

 dependent (unconnected) there. The fact 

 that this weakening is most pronounced and 

 that the detached condition occurs in the more 

 specialized members of the several groups is 

 passed by unnoticed. In generalized Lepidop- 

 tera, Hepialus, the Psychidse, Cassidse, etc., 

 these veins have basal connections, and 

 when free proximally, their dislocated basal 

 rudiments within the discal cell might well 

 have been accounted for. In the Ephem- 

 eridse, where the author finds his series 

 of free and primitive independents most 

 highly developed, they are, unfortunately, 

 most free in Callibostis, Bcetis, Chloson, etc. — a 

 bunch of genera representing the extreme 



