216 



CHAPTERS ON EVOLUTION. 



joints, and the Zoe'a becomes thus modelled (Fig. 135) into the exact 

 form of a Mysis or opossum- shrimp (Fig. 131). Finally, the single and 

 median eye disappears, the outermost of the two end joints of each 

 of the chest-limbs disappears, leaving these walking legs (seen so 

 plainly in shrimp, prawn, crab, and lobster) of single conformation ; 

 gills are developed within the chest, sense-organs appear, and the full 

 development of the prawn (Fig. 132) is then completed. Throughout 

 these varied stages it is not difficult to trace a panoramic succes- 

 sion of forms accurately reproducing the existing 

 degrees and forms of the crustacean class. The 

 early Nauplius (Fig. 133), the zoea or water-flea 

 stage (Fig. 134), the my sis-form (135), each pro- 

 duced in definite and advancing succession, pre- 

 sent us with a perfect picture of the evolution of 

 the prawn-race from lower crustacean life, and, 

 presumably also, of the evolution of all other 

 crustaceans belonging to the same rank and series 

 in the class. 



In summarising the results to which a study 

 of the development of the echinoderms and 

 crustaceans leads, there is to be recognised 

 the operation of the principles already more 

 than once insisted upon in the preceding pages, 

 namely, that community of descent is provable 

 by likeness in development, just as differences 

 or obliterations and alterations in development 

 are explicable on the grounds of adaptation and 

 change acting concurrently with the evolution and 

 progress of the race. Only by taking into account 

 these two principles, can the hard ways of develop- 

 ment be understood. The present subject is one 

 which may be regarded as lying thoroughly with- 

 out the province and power of any explanation 

 not founded upon evolution and upon the idea 

 that progressive change is part and parcel of 

 the order of nature. Admitting that the only 

 feasible explanation of these curious phases of development is to 

 be found in such an idea of nature's constitution, it seems folly to 

 deny that the general weight of evidence in favour of descent more 

 than counterbalances any difficulties which may present themselves 

 in connection with the exact determination of the lines along which 

 that descent has travelled. That larval or young forms are them- 

 selves liable to modification from various circumstances must be 

 admitted. This variation (to be hereafter studied in the insect-class) 

 of the young form, which we regard as representing the primitive 

 stock of the class, must unquestionably complicate the study of 



