74 SECTIONAL ADDRESSES 



therefore the dropping of Haeckel's terms ' palingenesis ' and ' coenogenesis,' 

 apparently because the former implies that characters which appear in 

 the adult stage are heritable, whilst the ccenogenetic characters are limited 

 in heritability to the larval stages. Since the evidence at his disposal led 

 him to believe that new characters enter the phyletic history only during 

 early ontogeny he proposed the terms ' palasogenetic ' for those ontogenetic 

 processes which functioned early in phyletic history and ' neogenetic ' for 

 those which came into action later. He differentiates structures in a 

 corresponding manner by using the terms ' palaeomorphic ' for primitive 

 types of structures and ' neomorphic ' for modified types of structures. 



Coenogenesis {the appearance of new characters at an early stage of 



development). 



Though Haeckel's main emphasis was upon recapitulation he realised 

 that certain factors were at work which tended to vitiate the developmental 

 record. Among these was the appearance, in larvae and embryos, of 

 features which were adaptations to the conditions under which these 

 immature organisms lived. He crystallised his observations by intro- 

 ducing the term ' coenogenesis ' for this phenomenon and by distinguishing 

 a ccenogenetic stage in development, which he regarded as having no 

 recapitulatory and therefore no phylogenetic significance. Just as with his 

 principle of recapitulation the advance in knowledge has entailed modifica- 

 tion, so is it also with the principle of coenogenesis, but to a much greater 

 degree. It may be noted in passing that in the ccenogenetic stage the 

 resemblance of the young to those of preceding generations is not wiped 

 out, but the new characters are superposed upon a combination of ancient 

 or primitive (palaeomorphic) characters, built up during the evolution of 

 the young themselves. 



Garstang seems to have objected to the term ' coenogenesis ' because 

 it implied that these larval characters exerted no influence upon the sub- 

 sequent growth stages in either development or evolution. Nevertheless 

 some ccenogenetic characters and the evolutionary changes they undergo 

 are confined wholly to early development, and apparently exert no 

 appreciable direct influence upon the later stages. This point was 

 indeed stressed by Garstang himself for certain adaptations to motile 

 life exhibited by larvae. Of these he says, ' the modification of the larva 

 in this way need not affect the organisation of the adult.' 



Fossil examples are perhaps less easily demonstrated. One clear 

 case, however, may be quoted from among the ammonoids in which 

 the protoconch undergoes evolutionary change. Those changes which 

 find systematic expression in the terms ' assellate,' ' latisellate ' and 

 ' angustisellate ' do not appear to have influenced the later developmental 

 and evolutionary changes in any way. Considerable differences are 

 recognisable between the protaspids of various trilobites and the proto- 

 conchs of gastropods. They also appear to have no effect upon the 

 subsequent development of these organisms. 



Some ccenogenetic characters may possibly have exerted a radical 

 influence upon subsequent growth and evolution, though they themselves 



