THE EVIDENCE FROM DEVELOPMENT. 217 



evolution and add to the difficulties of constructing a perfect pedi- 

 gree of the living world. The Pluteus larva of a sea-urchin and the 

 Bipinnaria larva of a starfish, thus differ in respect that the former 

 possesses a limy framework which is wanting in the latter. But such 

 distinctions do not in the least degree militate against the primary fact 

 underlying all such developments, namely, that the likenesses, not 

 merely of young forms, but in adult structure, are explicable only on the 

 theory of a common origin. Indeed, with the best of reason and logic, 

 it may be argued that, as a condition of evolution, we postulate the 

 occurrence of variations in the young stages as well as in the adult 

 form just as we should legitimately expect to find in living horses 

 the rudiments of those toes which the ancestors of the existing equine 

 race possessed. Thus " direct " development, such as we have seen 

 to occur in some starfishes and sea-cucumbers, whereby the young 

 pass directly into the form of the adult, and wherein the changes of 

 structure and appearance are suppressed, is a result of the adapta- 

 tion of the larvae to new ways of life. Rejecting this view, we should 

 have to fall back upon the anomalous position of maintaining that 

 there existed for one echinoderm a law of special creation, and for 

 another a law of descent a supposition which no logical mind will 

 accept, and which the grander idea of the uniformity of nature at once 

 dispels. As a final remark in connection with the sea-urchin class 

 and its transformations, we may add that the changes in form are 

 themselves progressive in nature. The five existing groups of this 

 class (sea-urchins, starfishes, sand-stars, sea-lilies, and sea-cucumbers) 

 are unquestionably modifications of a common plan of structure, and 

 they originate from a larva which is wonderfully similar throughout, 

 if we consider the diversities of adult structure which arise therefrom. 

 Further, if this larva were to be arrested in its development and to 

 represent a mature form in such an arrested stage, it would present a 

 striking resemblance to some of the lower worms and their allies ; this 

 fact alone pointing to the probable beginnings of the sea-urchin class 

 in a worm-stock. No less clearly do we see in the varying degrees 

 of organisation exhibited by adult echinoderm s, the same proof of 

 progressive advance and modification of an originally primitive type. 

 The forces and powers which, before our waking eyes to-day, evolve 

 a sea-urchin from its egg and easel-like larva, or a starfish from its 

 Bipinnaria, are, if we will only consider the wonderful nature of the 

 transformations involved, engaged in as evident and intricate a work 

 of evolution as those which have developed the varied twigs and 

 branches of the Echinoderm tree in the aeons of the past. 



The foregoing conclusions find, perhaps, plainer illustration in the 

 history of the Crustacean class, wherein exists a uniformity not so clearly 

 traceable although its original existence may not be doubted in the 

 early life of the echinoderms. The highest members of the Crustacea 

 are, as we have noted, the lobsters, crayfishes, shrimps, crabs, and 



