LIFE-HISTORY 487 



ment. In Ciona, for instance, the eggs are liberated at dawn 

 and the larvae are hatched the following night. They swim 

 about for one or two days and then undergo metamorphosis. 



Such a life-history illustrates several points very clearly. 

 (1) No anatomist, however skilful, could have discovered from 

 the adult Ascidian its Chordate affinities. The embryologist 

 sometimes gives a clue to the classifier. (2) In a very general 

 way it may be said that individual development (or ontogeny) 

 tends to recapitulate the main steps of racial evolution (or phy- 

 logeny). In most animals this is particularly true when we 

 consider the stages in " organogenesis " ; in other words, in the 

 up-building of an organ there is a marked tendency to pass 

 through a succession of stages which are permanently repre- 

 sented in a series of less highly evolved animals, representative 

 at least of ancestral forms. To speak of an animal " climbing 

 up its own genealogical tree " is to use somewhat rough popular 

 language, but it is pardonable perhaps because of its vividness. 

 But we cannot consider the life-history of a typical Ascidian 

 without feeling that the individual development tells in a con- 

 densed form the story of the origin of these strange sluggish 

 creatures from simple primitive Chordates. 



In this connection, however, it is necessary to remember 

 that larvee have often characters adaptive to their peculiar modes 

 of life ; and we must not be in a hurry to reconstruct the an- 

 cestral Chordate from the young stages of forms which have 

 certainly diverged far from the main track of vertebrate evolu- 

 tion. (3) The life-history illustrates individual degeneration 

 or " retrogressive metamorphosis ". " The larva," Professor 

 Herdman writes, " is comparable with a larval fish or a young 

 tadpole, and is thus a Chordate animal showing evident re- 

 lationship to the Vertebrata ; while the adult is in its structure 

 non-Chordate, and is on a level with some of the worms, or with 

 the lower Mollusca, in its organization, although of an entirely 

 different type." Of course this is true, and yet is there not a 

 risk of exaggerating the degeneration. The larva is of a 

 higher type, it is a Chordate larva ; the adult Ascidian is of a 

 lower type, for it loses all the essential Chordate characteristics 

 except the ventral heart, the gill-slits being too much modified 

 and disguised to count. At the same time, one must not think 

 of an Ascidian as degenerate in the sense of being undifferenti- 



