24 VERTEBRATE ZOOLOGY 



hood causes the pushing back into the earlier stages of some of the 

 characters that may have had a later phylogenetic origin, and also 

 calls forth the development of new characters that are merely adapta- 

 tions for larval life. Characters that are considered truly ancestral 

 are known as palingenetic, and those that are mere interpolations 

 for purposes of larval adaptation are known as ccenogenetic. A good 

 example of a palingenetic life history is that of the common frog, 

 but even here there are certain larval characters that are purely 

 csenogenetic. In some of the non-aquatic frogs in which the eggs are 

 not laid in the water the typical cycle is greatly foreshortened through 

 the omission of the larval stages; the young hatches out as a little 

 frog with only a mere stump of a tail left. This type of foreshortening 

 the life history is known as tachygenesis. 



It will appear from what has just been said that some ontogenies 

 may be fairly reliable recapitulations of phylogenies, but that others 

 are entirely unreliable as guides in phylogenetic matters. The ques- 

 tion may well be asked as to whether we are in a position to decide 

 in any given case whether the embryonic history is truly palingenetic 

 or not. In answer to this we may say that the evidence of embry- 

 ology is valuable only when it is supported by a study of comparative 

 anatomy and palaeontology. 



One point about the recapitulation theory that is seldom clearly 

 apprehended has been clearly stated by Lillie: 



"If phylogeny is to be understood to be the succession of adult 

 forms in the line of evolution, it cannot be said in any real sense 

 that ontogeny is a brief recapitulation of phylogeny, for the em- 

 bryo of a higher form is never like the adult of a lower form, 

 though the anatomy of embryonic organs of higher species re- 

 sembles in many particulars the anatomy of homologous organs 

 of the adult of the lower species. However, if we conceive that 

 the whole life history is necessary for the definition of a species, 

 we obtain a different basis for the recapitulation theory. The 

 comparable units are then entire ontogenies, and these resemble 

 one another in proportion to the nearness of relationship, just as 

 the definitive structures do. Thus in nearly related species the 

 ontogenies are very similar; in more distantly related species 

 there is less resemblance, and in species from different classes 

 the ontogenies are widely divergent in many respects." 

 It must not be forgotten, however, that the ontogenies of closely 



