ORGANIC EVOLUTION — PHYSICAL 23 



undergo equal retrogression. Various causes prevent 

 the retrogression from being everywhere equal, since 

 the conditions under which retrogression takes place 

 cannot be an exact reversal of those under which 

 evolution occurred ; for instance, the retrogressing wing 

 of the apteryx is attached to a body which has not 

 undergone a "pari passu retrogression. Moreover, even 

 while the wing was undergoing evolution, parts of it 

 must have undergone retrogression; this and that 

 structure or part of a structure in it, useless in a wing, 

 but useful in the organ which preceded it, must have 

 tended to disappear at the same time that other struc- 

 tures or parts of structures, more useful in a wing than 

 in the organ that preceded it, tended to increase ; the 

 two warring forces of natural selection and atavism 

 thus operating to bring the wing and the organism to 

 which it belongs into completer harmony with the 

 environment. Thus all the parts of the degenerated 

 wing of an apteryx cannot have undergone equal retro- 

 gression, and therefore the resemblance between the 

 degenerated wing and the ancestral form in any of its 

 stages cannot be very close. 



It follows, if the above theory be correct, that while 

 it is not possible by means of selection, natural or arti- 

 ficial, to bring about rapid and extensive evolution, since 

 such evolution must soon be checked by the increasing 

 tendency towards far-reaching reversion, it is possible 

 by means of selection to bring about rapid and exten- 

 sive, indeed unlimited, retrogression. This hypothesis, 

 however, is at variance with accepted doctrines. Mr. 

 Herbert Spencer says — 



"Leaving open the question whether, in indefinite 

 time, indefinite modifications may not be produced; 

 experience proves that within assigned times, the 

 changes wrought in races of organisms by changes of 



