88 EVOLUTION OF LIVING ORGANISMS. 



vertebrate, yet the young free-swimming larva has all 

 the characteristics of the Vertebrate phylum, with its 

 dorsal central nervous system, gill-slits, axial skeletal 

 rod or notochord, and tail. The cirripedes or barnacles 

 likewise have taken to a fixed habit in adult life, and have 

 lost a most all resemblance to the Crustacea from which 

 they have been derived, as is shown by the larval stages 

 typically crustacean. Other common instances of de- 

 generation are afforded by parasites. Often they be- 

 come simplified beyond recognition, but betray their true 

 affinities in their development. Many parasitic Crustacea 

 of the order Copepoda lose practically all trace in the 

 adult condition of the characteristic appendages used in 

 the normal free-living forms for locomotion or for seizing 

 and cutting up their food. Indeed, parasitic animals, 

 able as they are to absorb the nourishment directly from 

 their host, generally tend to lose not only organs of loco- 

 motion and of special sense, but even the alimentary 

 canal, and spend all their energies in producing enormous 

 numbers of young to infect new hosts. The loss of the 

 organs of flight in flightless birds and insects, and of the 

 eyes in animals living in caves or the dark depths of the 

 ocean is also due to degeneration. 



Degeneration, in fact, is a widespread phenomenon 

 among animals and plants, and leads to the loss of 

 any special structures mental or bodily which the organ- 

 ism no longer needs in the particular environment for 

 which it has become adapted. It is a return from a com- 

 plex to a simpler organisation ; but not to a truly primi- 

 tive or ancestral condition, for the path of retrogression 

 is generally very different from that followed by the 

 original progressive evolution. Now it is one of the great 

 merits of the doctrine of evolution by natural selection, 

 that it accounts for this simplification as easily as for the 

 development of complexity. For both progressive and 

 retrogressive mutations occur. Variation takes place 

 both in the + and in the - direction, and selection of the 



