The Theory of Evolution 69 



rudimentary legs. In these, of course, the 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 egg 

 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 penaeus, 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 zoea. 

 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 zoea is the ancestor of the decapods. The later 

 development of the zoea shows, however, that it cannot 

 be such an ancestral form, for, in order to reach the. 



