60 THE CONSTITUTION OF THE GASTROPOD PROTOCONCH, 



explicitly put by Fritz Muller and Haeckel in the statement 

 that the ontogeny of an organism is a recapitulation of its 

 phylogeny. There are certainly many characteristic phases 

 in the ontogeny, especially of the higher animals, finger- 

 posts, which point the road that has been travelled, but these 

 records constantly fail us when we look for information as to 

 the more recent stages in the journey. Thus, in the ontogeny 

 of the Horse, we find no Eoliipj^us stage ; in that of birds, no 

 hesperoniis or Ichthyornis stage. In the Platypus, on the 

 otJier hand, we find traces of a toothed ancestor in the onto- 

 genetic records ; and among invertebrates, in Cymatium pa?-- 

 Jiinsonianum we find abundant trace of the sculptured ances- 

 tor, C. torirostris, in the early whorls (see later, p. 74). It is 

 to be noted, however, that in all those cases where trace of 

 some recent ancestor is preserved in the record, it is present 

 at a time when the organism is practically or actually in pos- 

 session of all the adult features. 



In general, it may be stated that only the complete adult 

 organism is sufficiently strong to leave an impress upon the 

 embryonic ontogenetic record. Thus the reptilian affinities 

 of the Aves are recognisable in the ontogeny, but not so the 

 teeth of a more recent ancestor. To state a hypothetical case, 

 suppose that a branch of the Gastropoda should become so 

 modified as to no longer be classed with the Mollusca, we 

 would expect to find traces of its molluscan ancestry in its 

 ontogeny, but it is possible that its origin from the Gastro- 

 poda would not be clear, and it is highly improbable that 

 from the ontogenetic record we should be able to place the 

 ancestors as Opisthobranchs, Prosobranchs or Pulmonates. 

 ID seems, then, that strong though the power of retention of 

 ancestral features may be, it is insufficiently strong to retain 

 permanently any trace of those minor fluctuations which have 

 undoubtedly occurred in the evolution of the species ; con- 

 versely, strong though the tachygenetic tendencies may be to 

 modify the ontogeny of an organism, it is not until they 

 affect the whole organism in a fundamental manner that they 



