RECAPITULATION 65 



108. The regression and ultimate disappearance of struc- 

 tures, especially large and important structures, has often 

 been attributed to that form of Natural Selection which is 

 known as reversed selection, " the selection which affects not 

 increase of an organ, but decrease of it." No doubt reversed 

 selection does to some extent hasten the elimination of 

 burdensome and injurious structures by selecting for survival 

 individuals in whom as a consequence of reversion the struc- 

 ture is poorly developed, but we shall see presently that other 

 selective means have been provided by nature so efficient that 

 reversed selection has little scope to act in such cases. It 

 plays a great part nevertheless. Imagine again our series of 

 individuals A. . . . M. who have evolved a certain structure 

 in a certain direction by an uninterrupted series of pro- 

 gressive variations. If N. completely recapitulates M., he 

 may afterwards vary in an endless number of different ways. 

 Amongst others he may double back, as it were, on his own 

 development, so that in him the structure is less perfect at 

 the end of his development than it was immediately earlier. 

 If this process be continued in subsequent generations, then 

 in the remote descendants the structure will be less perfect 

 in the adult than in the immature individual. Indeed it 

 may be quite absent in the adult. Numerous examples of 

 this process may be found in nature. For instance, the 

 caudal appendage is better developed in the human embryo 

 than in the adult. Still better illustrations are afforded by 

 the disappearance in adult life of the larval or embryonic 

 structures of numerous animals, for example, frog, ascidian, 

 and many insects. In all these instances, as in cases of latency, 

 there is no real regression in the sense in which we have 

 used the term. The individual does not regress as compared 

 to his progenitors, but only as compared to his younger self. 

 The germ-plasm does not lose a tendency. Even the indi- 

 vidual, if his development be considered as a whole, loses 

 nothing. On the contrary, his development gains a pro- 

 longation. 



109. The nature of the work done by reversed selection 

 will be rendered clearer if we examine a couple of well- 

 known and oft-quoted examples. Eyes have been evolved 

 through the action of Natural Selection. In animals living 

 in total darkness, in certain cave-dwellers, for example, the 

 eye has become, not only useless, but worse than useless, 

 since it is an extremely prominent and tender, and therefore 

 vulnerable part of the organism. In some of these animals 

 the eye is better developed in the immature than in the 



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