564 



THE AMERICAN XATURALIST [Vol XI> 



agascar. They are diurnal, contrary to the general rule among^ 

 volant mammals, though they prefer the morning and evening for 

 their periods of activity. 



Summary 



1. Volant evolution has occurred seventeen times among ver-^ 

 tebrates, ten of which are merely adaptations for more or less pro- 

 longed soaring leaps, while in seven instances in all probability 

 true flight has been developed. 



2. Soaring implies, with but one exception, the development 

 of a fold of skin along thecreature's flanks supported in one instance 

 by the extension of the ribs beyond the body wall, but generally 

 stretched between the fore and hind limbs. This fold is often sup- 

 plemented l)y others in front of the fore limbs and between the hind 

 limbs sometimes involving the tail. 



3. True flight always implies a more or less profound modifi- 

 cation of the fore limbs which become, as a consequence, unsuited 

 to ordinar\' progression. True flight has been developed once in 

 each of the classes of strictly air-breathing vertebrates, and prob- 



4. Willi tlir f\(r|)tii)ii (if the li-lic-, -oaring implies also pres- 

 ent or aiKT-irai ail)oM ai a- la; )ta I ion ami this may a{)ply as well to 

 the Tnir tli<"r.. It i. . rrtaiiily true of tin- l.aN, p'ossiblv true of the 

 bird.. bntoftlir,.trnMlartvU,„H. .-aniioi b.- ...rtain. 



:.. P..-si.jr. til." priiuary ino.lilication. ul.icli .on.tit.it." the 

 machinery of flight, other porti.Mi. of tlir bo* K . especially t he 

 nervous system, the sense and the nutritive organs, may exhibit 

 secondary volant characteristics. These, as with the primary 

 modifications, are in direct proportion to the powers of flight. 



