LAWS OF VARIATION 193 



some manner excited, but it is the will of man which 

 accumulates the variations in certain directions; and it is 

 this latter agency which answers to the survival of the 

 fittest under nature. 



Effects of the increased Use and Disuse of Parts, as 

 controlled by Natural Selection 



From the facts alluded to in the first chapter, I think 

 there can be no doubt that use in our domestic animals 

 has strengthened and enlarged certain parts, and disuse 

 diminished them; and that such modifications are inher- 

 ited. Under free nature, we have no standard of com- 

 parison by which to judge of the effects of long-contin- 

 ued use or disuse, for we know not the parent-forms; 

 but many animals possess structures which can be best 

 explained by the effects of disuse. As Professor Owen 

 has remarked, there is no greater anomaly in nature than 

 a bird that cannot fly; yet there are several in this state. 

 The logger-headed duck of South America can only flap 

 along the surface of the water, and has its wings in 

 nearly the same condition as the domestic Aylesbury 

 duck: it is a remarkable fact that the young birds, ac- 

 cording to Mr. Cunningham, can fly, while the adults 

 have lost this power. As the larger ground-feeding bird^ 

 seldom take flight except to escape danger, it is probable 

 that the nearly wingless condition of several birds, now 

 inhabiting or which lately inhabited several oceanic isl- 

 II ands, tenanted by no beast of prey, has been caused by 

 disuse. The ostrich indeed inhabits continents, and is 

 exposed to danger from which it cannot escape by flight, 

 but it can defend itself by kicking its enemies, as effi- 

 ciently as many quadrupeds. We may believe that the 



— Science — 9 





