CHAP, i.] INTRODUCTION. 13 



species are kept up, representative as it were of each 

 other. When there is such a representation, the 

 minimum of one species usually commences before 

 that of which it is representative has attained its 

 correspondent minimum. Forms of representative 

 species are similar, often only to be distinguished by 

 critical examination." l 



As an illustration of what is meant by the law of 

 ' representation,' I may cite a very curious case men- 

 tioned by Mr. Verril and Mr. Alexander Agassiz. On 

 either side of the Isthmus of Panama the Echinoderm 

 order Echinidea, the sea-urchins, are abundant ; but 

 the species found on the two sides of the Isthmus 

 are distinct, although they belong almost universally 

 to the same genera, and in most cases each genus is 

 represented by species on each side which resemble 

 one another so closely in habit and appearance as to 

 be at first sight hardly distinguishable. I arrange 

 a few of the most marked of these from the Carib- 

 bean and Panamic sides of the Isthmus in parallel 

 columns. 



EASTERN FAUNA. WESTERN FAUNA. 



Cidaris annulata, GRAY. Cidaris thouarsii, VAL. 



Diadema antillarum, PHIL. Diadema mexicanum, A. AG. 



Echinocidaris punctulata, DESML. Echinocidaris stellata, AG. 



Echinometra mickelini, DES. Echinometra van brunti, A. AG. 



viridis, A. AG. ,, rupicola, A. AG. 



Lytechinus variegatus, A. AG. Lytechinus semituberculatus, 



A. AG. 



Tripneustes ventricosus, AG. Tripnemtes depressus, A. AG. 



Stolonoclypiis ravenellii, A. AG. Stolonoclypus rotund us, A. AG. 



Mellita testudinata, KL. Mellita longifissa, MICH. 



1 Edward Forbes, Report on ^Egean Invertebrata, op. cit. p. 173. 



