484 SCIENCE PROGRESS. 



expressed the truth of the matter when he stated that 

 ancestral stages were Hable to be dropped out or omitted 

 in embryonic development, to be hidden or replaced by 

 new characters in larval development. We have long" 

 been accustomed to use technical terms to distingfuish 

 ontogenetic characters which are probably or evidently re- 

 capitulative, from those which have evidently been recently 

 evolved in the larva or embryo : we call the former 

 palingenetic, the latter kainogenetic. What is new there- 

 fore in Mr. Sedgwick's paper is not the proposition that 

 ancestral structure is not always recapitulated in the early 

 stages of individual history, or that many embryonic or 

 transitory characters are not ancestral. The main object 

 of the paper is to support a particular explanation of the 

 apparent capriciousness which is observed in the persistence 

 or disappearance of ancestral organs and characters. This 

 explanation is that the retention of an ancestral condition 

 in the series of embryonic changes indicates that this con- 

 dition was once a larval condition, that is to say, was 

 retained as a necessary adaptive character in a free-living 

 larval stage of the animal, after the new adult character had 

 been evolved. Afterwards the larval stage ceased to be 

 free, the young animal being confined either in an egg-shell 

 or in the maternal uterus until its metamorphosis had been 

 accomplished, when it was hatched or born with a structure 

 more or less exactly similar to that of the adult. On the other 

 hand, according to Mr. Sedgwick, when a change occurs 

 in evolution in an animal which has no larval stage, in other 

 words when a new adaptation is produced which is related 

 to conditions existing throughout the independent life of 

 the organism, then this variation or modification affects the 

 whole constitution of the individual, has a retrospective 

 influence on the embryonic development, and comes to be 

 developed directly with no, or scarcely any, trace at any 

 embryonic stage of the structural condition which must 

 have preceded it in the line of descent. The two most 

 marked instances of these contrasted modes of develop- 

 ment cited by Mr. Sedgwick are (i) the existence of gill- 

 clefts and gill-arches in the embryos of all air-breathing 



