CIEAP. 1.] INTRODUCTION. 13 
e 
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.””’ 
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 Hehinidea, 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. 
Hastern Fauna. WESTERN Fauna. 
Cidaris annulata, GRay. Cidaris thouarsii, Vat. 
Diadema antillarum, Put. Diadema mexicanum, A, Ac. 
Echinocidaris punctulata, Desmu. LE chinocidaris stellata, Ac. 
Echinometra michelini, Des. Echinometra van brunti, A. AG. 
o viridis, A, AG. rupicola, A. Aa. 
Lytechinus variegatus, A. AG. Lytechinus semituberculatus, 
A. Ag. 
Tripneustes ventricosus, AG. Tripneustes depressus, A. AG. 
Stolonoclypus ravenellii, A. AG. Stolonoclypus rotundus, A. Ac. 
Mellita testudinata, Ku. Mellita longifissa, Micn. 
* Edward Forbes, Report on Aigean Invertebrata, op. cit. p. 173. 
