190 CALIFORNIA ACADEiMY OF SCIENCES. 



is necessary to keep in mind two general influences, the 

 internal forces of growth regardless of the effect to 

 be produced, and external selection, which looks only at 

 the effect. We have now seen how selection would tend 

 to produce the most striking effects of form and color of 

 marking upon those parts which are most exposed, and 

 especially those parts which are most constantly in mo- 

 tion — the head, throat, wings, tail, rump and under- 

 tail coverts. At the same time, other things equal, 

 these effects would always be produced in the easiest and 

 most natural way, rather than in direct opposition to 

 the laws of growth. 



With regard to the wing markings, the laws of devel- 

 opment apply when the individual feathers alone are 

 taken into consideration, but the general effect of wing 

 marking is produced in a great variety of ways which 

 has no obvious relation to laws of growth or mechan- 

 ical forces. Wing markings are generally white in 

 strong contrast to some very dark shade, generally 

 brown, and are obviously recognition marks of some 

 sort. It might be thought that the white was due to de- 

 generation if it uniformly occurred at the tips of the 

 feathers where the wear is the greatest; but in point of 

 fact, while it is very often situated thus, it also occurs 

 at the base or through the middle of the feather. Thus 

 the cedar wax-wing {Ampeliscedrorum) has no white wing 

 bars, while the Bohemian wax-wing {Ampelis garrulus) 

 which is so closely related, has a wing-bar formed by 

 the white spots at the terminus of the primary coverts, 

 another on the tertiaries, and a vertical line down the 

 wing formed by the white and yellow spots on the outer 

 edge of the tips of the primaries. It surely cannot be 

 held that any internal law of growth, or external me- 

 chanical force which produced such a variety of effects 

 in the one species could have been almost or wholly in- 



