The Theory of Evolution 69 
rudimentary legs. In these, of course, che rudiments of legs 
must appear in the embryo, but in the legless forms even the 
beginnings of the legs have been lost, or at any rate very 
nearly so. 
Outside the group of vertebrates there are also many 
cases that have been interpreted as embryonic repetitions 
of ancestral stages, but a brief examination will suffice to 
show that many of these cases are doubtful, and others little 
less than fanciful. A few illustrations will serve our pur- 
pose. The most interesting case is that given by the history 
of the nauplius theory. 
The free-living larva of the lower crustaceans — water- 
flees, barnacles, copepods, ostracods — emerges from the ege 
as a small, flattened oval form with three pairs of append- 
ages. This larva, known as the nauplius, occurs also in 
some of the higher crustaceans, not often, it is true, as a free 
form, as in penzeus, but as an embryonic stage. The occur- 
rence of this six-legged form throughout the group was 
interpreted by the propounders of the nauplius theory as 
evidence sufficient to establish the view that it represented 
the ancestor of the whole group of Crustacea, which ancestor 
is, therefore, repeated as an embryonic form. This hypothe- 
sis was accepted by a large number of eminent embryologists. 
The history of the collapse of the theory is instructive. 
It had also been found in one of the groups of higher 
crustaceans, the decapods, containing the crayfish, lobster, 
and crabs, that another characteristic larval form was 
repeated in many cases. This larva is known as the zoéa. 
It has a body made up of a fused head and thorax carrying 
seven pairs of appendages and of a segmented abdomen of 
six segments., The same kind of evidence that justified 
the formulation of the nauplius theory would lead us to infer 
that the zoéa is the ancestor of the decapods. The later 
development of the zoéa shows, however, that it cannot 
be such an ancestral form, for, in order to reach the 
