THE EVOLUTION THEORY 135 



other changes take place, transforming the Nauplius into a Zosea. 

 The median eye is replaced by a pair of eyes, and the three pairs 

 of appendages become six. 



Continued growth and moulting produces 

 the second Zosea stage, which approximates 

 more and more that of the adult animal. 

 The number of appendages is again in- 

 creased, but whilst formerly all served the 

 purpose of locomotion, they now gradually 

 assume separate definite functions. The 

 two first appendages become antennae, the 

 next develop into jaws, whilst the other be- 

 come rowing-legs. The abdomen is better FIG - 4 

 developed and the eyes become typical lateral 

 outgrowths of the head. 



Another moult brings the Zosea to the Mysis stage. It has 

 now thirteen pairs of appendages, of which the third, fourth, and 

 fifth are used as masticatory organs. The legs are assisted in 

 locomotion by the abdomen, the most posterior segment of which 

 has become transformed into a frond. This larval form is 

 especially interesting because we find in the order Schizopoda 

 a family of the Mysidse the members of which permanently 

 remain at the Mysis stage. A last moult concludes the com- 

 plicated metamorphosis, giving the Penseus its final shape. 



Though the opinion is no longer held by modern naturalists 

 that adult animals of the Nauplius type existed among the oldest 

 ancestors of the crustaceans, and though it is now doubted for 

 several reasons whether a mature Nauplius ever existed, their 

 regular appearance in the evolution of all crustaceans can only be 

 explained in the sense that the Nauplius is the primary larval 

 form from which the living crustaceans have descended. 



There is no better test for the accuracy of a theory than that 

 it enables us to make certain predictions which are subsequently 

 corroborated by observation. One of the most brilliant triumphs 

 of science is the discovery of the planet Neptune. As early as 

 1828 Bessel contended, on the basis of certain slight irregularities 

 in the path of Uranus, that they were probably caused by some as 

 yet far-distant planet. Independently of each other, yet almost 

 simultaneously, the famous English astronomer Adams, and 



