The Recapitulation Theory. 91 



Perhaps the most convincing of all cases for a Lamarckian 

 interpretation of recapitulation is that of the parasitic Copepod, 

 Achtheres ambloplitis, where as MacBride remarks, "we may almost 

 say that the ancestor is known." The adult is a parasite on the 

 gills of the rock-bass, and is a sac-like organism devoid of all 

 semblance of Copepod structure, yet the animal passes through a 

 brief larval stage which anyone would recognize at once as 

 representing a typical Copepodan genus. Whatever explanation 

 applies to this remarkably clear case must also apply mutatis 

 mutandis to the whole series of larval stages which represent the 

 remoter ancestry in other organisms. We think this case furnishes 

 one of the clearest evidences of the sharp contrast we are 

 endeavouring to draw between recapitulatory and mutational, or in 

 other words, between organismal and karyogenetic characters. Yet 

 it is admitted that even in this Copepodid larva there are modifica- 

 tions from the typical details of Copepod structure. But they 

 follow the usual lines of diminution in size of the larva and 

 consequent reduction in the number of metameric parts. Another 

 factor which tends to obscure ancestral stages is the well-known 

 principle of telescoping, or the earlier appearance of embryonic 

 organs, which has been called heterochrony by Lankester, or 

 tachygenesis by French writers (Perrier and Gravier, 1902). That 

 embryonic and larval stages can also undergo special modifications 

 of an adaptational nature is another well recognized principle which 

 tends to obscure ancestral relationships. 



Many of the battles of recapitulation have been fought over the 

 frog, so we may briefly examine his case. Some embryologists 

 have gone so far as to deny any ancestral significance to the 

 tadpole. While the other extreme view, that all larval, characters 

 of the frog have ancestral significance, is certainly disproved ; yet 

 the truth evidently lies in a recognition of the fact that in the 

 tadpole as in most other larvae, there are some recapitulatory 

 or palingenetic characters and some adaptational or ccenogenetic 

 characters. The obvious recapitulatory characters are the fish-like 

 tail, gill arches and blood system. It seems impossible to avoid 

 the conclusion that these were once terminal developmental stages, 

 in the frog's fish-ancestor. On the other hand Boulenger (1918) 

 has pointed out that tadpoles have had an evolutionary history of 

 their own and have developed special adaptational features. He 

 enumerates (Boulenger 1897) "The horny beak and circular lip 

 with its horny armature, the spiraculum, the enclosure of the fore- 



