Mutations and Evolution. 
231 
Zoologists are generally agreed that the nauplius and zoaea 
larvae of Crustaceans and the veliger larva of Molluscs represent 
ancestral stages ; similarly, that the trochophore larva of Annelids 
and Molluscs shows these two very divergent groups to have been 
derived from a common ancestor. MacBride would go further and 
say that the trochophore is a smaller and somewhat reduced or 
specialized representative of that ancestor. According to the 
interpretation here adopted, the various larvae at one time 
represented terminal stages in their respective life-cycles, and by 
subsequent evolution they were not modified out of existence or out 
of recognition through germinal changes, but became subterminal 
through the addition of later stages to complete the ontogeny. In 
a field where every larva has its ancestral significance, it is almost 
invideous to single out particular cases for illustration. The tadpole 
of Ascidians, which gave the first hint of the Chordate affinities of 
this group, is a striking instance. Only the degeneration and special 
adaptations resulting from the adoption of a sedentary life can 
account for their remarkable transformation, and we fail to see how 
it can be adequately explained except on a neo-Lamarckian basis. 
An equally irrefutable case of recapitulation is that of the feather- 
star Antedon, whose egg developes into a free-swimming larva which 
later becomes fixed by a stalk. This fixed stage corresponds with 
the modern genus Pentacrinus. The adult feather-star developes 
cirri and, losing its stalk, becomes free again. Can anyone pretend 
to believe that this remarkable series of transformations does not 
recapitulate the history of the race ? And, if so, how can one avoid 
the conclusion that the life-cycle has been lengthened? 
A type of recapitulatory character corresponding to the 
appearance of evanescent stages in the gametophytes of the 
Gymnosperms, is found in certain nauplius larvae. MacBride (l.c. 
p. 204) regards the nauplius as representing the common ancestor 
of all Crustacea. He says, “ When the larva does nothatch out as 
a nauplius, a cuticle is produced and shed by the embryo whilst 
still within the egg shell when it reaches the nauplius stage, thereby 
showing that formerly this stage must have been passed through in 
the open, in the ancestors of the forms in which it is now purely 
embryonic.” Again, in the oyster, which has no foot and becomes 
permanently attached by one valve of its shell, the veliger larva 
nevertheless possesses a functional foot which is therefore a 
recapitulatory character. 
