EVOLUTION OF THE COLORS OF BIRDS. 25 



periods of life, which progressively pushes back the de- 

 velopment of a rudiment to a more and more remote 

 embryonic stage of growth; and the second is the event- 

 ual failure of the principle of inheritance itself. For, 

 'whether or not we believe in Paiigenesis, we cannot but 

 deem it in the highest degree improbable that the influ- 

 ence of heredity is of unlimited duration.' ' The former 

 of these two factors is the same as Prof. Cope's principle 

 of retardation, while the latter has also been independ- 

 ently stated by Cope, as will be seen in a subsequent 

 quotation. There are, then, five theories besides pam- 

 mixis by which degeneration of parts may be explained, 

 viz: (1) reversal of selection, (2) economy of energy, (3) 

 disuse, (4) retardation, and (5) the failure of heredity. 

 In some cases the loss of an organ or member is an ad- 

 vantage, and then natural selection may aid in elimina- 

 ting it. An instance of this is to be found in insects 

 inhabiting islands, which would be blown to sea and 

 perish if they possessed the power of flight, and in which 

 the wings are aborted or entirely absent. There are 

 many cases, however, in which there is no such direct 

 advantage to be discovered in the reduction of a part. 

 To some of these cases the principle of economy enun- 

 ciated by Darwin might well apply. This principle 

 assumes that the organism has a given amount of force 

 to expend, and that if one part be useless the growth 

 force which has been expended in maintaining it will be 

 diverted to some other channel. Thus birds, like the 

 ostrich, in which the wings are aborted, have legs pro- 

 portionately powerful. The third principle is disuse. 

 It is universally admitted that whenever a part is not 

 used during the lifetime of an individual it degenerates 

 to a certain extent. The principle of disuse merely 

 assumes that such acquired degeneration can be inher- 

 ited. Cope enunciated the fourth and fifth principles, 



