18 STUDIES IN EVOLUTION 



or modification of superficial structures, and also through the 

 decadence of organs, has now been noticed. The spine may 

 thus be taken as a unit for comparison, and its various stages 

 of growth, which were shown to have a definite sequence, 

 may be used in correlation to determine relatively the degree 

 of spine specialization attained by any organism. Further- 

 more, enough data have been already given to lead to the 

 suspicion that spines may represent the limits of ornamental 

 or superficial differentiation or variation. At this point in 

 the discussion this statement must be considered as more 

 suggestive than conclusive. The proof of its reality will be 

 more clearly shown later on. 



Ontogeny of a Spinose Individual. 



With few exceptions the embryonic and larval stages of 

 all organisms are devoid of specialized surface features. In 

 other words they are without ornament and without weapons. 

 The exceptions to this rule seem to be readily explained 

 under the princij)les of larval adaptations and accelerated 

 development. Cases of the latter kind, therefore, can hardly 

 be considered as exceptions, since they represent, not real 

 larval features, but former adult characters which have been 

 pushed back or which develop earlier so as to appear even- 

 tually in the larval or later embryonic stages. In the very 

 earliest stages of embryonic development, the truth of the 

 first statement becomes obvious, and accordingly the pro- 

 tembryonic, mesembryonic, metembryonic, neoembryonic, and 

 typembryonic stages are without surface ornaments or spines. 



Among Mollusca, the protoconch, periconch, and prodis- 

 soconch, or the early larval shells, are smooth and without 

 ornament. Even the prodissoconch of very highly spinose 

 species, as in Spondylus^ is as smooth as that of the plainest 

 species of Ostrea, Anomia, Avicula, etc. Likewise, the proto- 

 conch of the most specialized or most retrograde cephalopod 

 is perfectly plain. In the nepionic stages the spiny Murex 

 is without spines. In the Brachiopoda the protegulum, or 

 early larval shell, is always without sculpture; while the 



