474 THE GENERAL PRINCIPLES OF ZOOLOGY 



it lies in what may be called retrograde metamorphosis, of 

 which some of the Crustacea furnish excellent examples. There 

 the adult in some parasitic species of Copepoda and Cirripedia 

 has become very degenerate in structure ; the usual crustacean 

 appendages are almost entirely lacking ; eves and other sense 

 organs are absent, and the body is little more than a swollen 

 sac capable of absorbing nutriment and of forming germ-cells. 

 Yet these animals pass through the usual larval stage common 

 to the majority of the Crustacea, provided with jointed append- 

 ages and with eyes and other sense organs. Thus the adult 

 is clearly descended from ancestral species in which such 

 structures were present and functional, while in its present 

 parasitic mode of life they are no longer required. 



There is another subject in the field of the philosophy of 

 biology which has occasioned much discussion and has been 

 the direct outcome of the various theories advanced to explain 

 the action of evolution ; this is the problem of heredity, which 

 demands a brief notice here. Heredity has been defined as " the 

 organic relation between successive generations." It is a truism 

 that like tends to beget like. We are not surprised that in non- 

 sexual reproduction the new individuals formed are like the 

 parent in all essential respects, for here the offspring are mani- 

 festly directly continuous with the parent ; they are portions of 

 it which may or may not become separated from it ; any varia- 

 tion under such conditions could not be accounted for. This is 

 well illustrated by plants which reproduce by runners or can be 

 made to develop from cuttings or grafts ; by animals which re- 

 produce through simple division, as in the Protozoa, or through 

 budding, as in many sponges and Cnidaria, some flatworms, all 

 the Bryozoa, some jointed worms, and many of the Tunicata. In 

 sexual reproduction, too, like tends to beget like so far as the 

 more fundamental characteristics of the species are concerned, 

 but the offspring may exhibit qualities which have come not 

 from the parents but from more remote ancestors ; such in- 

 stances of atavism, the recurrence of more or less remote ances- 

 tral traits, are not uncommon, especially among domesticated 

 plants and animals. Every one knows that most cultivated 

 fruits will not produce their kind from seeds, which are the 

 result of sexual processes, but only from cuttings, grafts, or 



