424 SCIENCE PROGRESS 



find that insects exemplify a sudden outburst of a group in 

 the Carboniferous, immediately subsequent to the unusually 

 rigorous desert-conditions of the Old Red Sandstone, which 

 were equally instrumental in inducing the evolution of air- 

 breathing amphibians from paddle-finned fishes, or the air- 

 breathing Scorpions and Anthracomarti from their purely 

 aquatic allies. The pools of stagnant water would eventually 

 dry up altogether and only those individuals which could 

 reach another sheet of water would survive to perpetuate 

 their race. During the process of stagnation ^ and evaporation 

 it is not difficult to imagine that those forms in which the 

 tracheal gills were more extensively developed, and sufficiently 

 stiffened to prevent collapse, might be able to leap out of 

 the water, and if this happened it seems obvious that the moist 

 membranes would be able to absorb oxygen from the air. 

 Such invigorating leaps would gradually become extended 

 into short flights (analogous to those of flying-fish), by means 

 of which the tracheal membranes would become stouter and 

 stronger, and the necessary muscles would soon become de- 

 veloped in correlation with this new function, whilst at the 

 same time the original respiratory function of the tracheal gills 

 would still be actively exercised. 



This picture is by no means so fanciful as it might appear 

 to be at first sight, for it affords a plausible explanation ot 

 a possible gradual change from tracheal gills to gliding-planes 

 and so to true wings, without implying any break in continuity 

 of function. The transition from tracheal gills to wings can 

 even be observed at the present day in Ephemerid larvae, 

 such as that of Chloeon, in which both the wings and the 

 6-"] pairs of tracheal gills entirely agree in their mode of 

 origin ; indeed some of the tracheal gills in this form are at 

 one period of their development even larger than the hind 

 wings (fig. 9), and the branching of the veins in the wing 

 corresponds very closely to the branching of the tracheae 



' It is interesting to find that stagnant water seems to exercise a marked 

 influence in increasing the extent of variabihty ; this has recently been estabhshed 

 for Cyclops by Dr. Esther Byrne (Fresh-water Species of Cyclops of Long Island, 

 Cold Spring Hufbour M onographs No. VII. 1909). In this monograph, which 

 is based on several years' work, the author finds that "variation of a varietal type 

 is strongly developed, but much more so in some species than in others ; // attains 

 its maximum in the forms inhabiting stagnant luatcrs^ which can only exist at all 

 by the power of readily adapting themselves to environment.'' 



