Page:The Depths of the Sea - Wyville - 1873.djvu/45

. I.] 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 mentioned 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 Caribbean and Panamic sides of the Isthmus in parallel columns.

Eastern Fauna. Western Fauna. Cidarls annidata, Gray. Cidaris thouarsii, Val. Diadema antillarum, Phil. Diadema mexicanum, A. Ag. Echinocidaris punctulata, Desml. Echinocidaru stellata, Ag. Echinometra michelini, Des. Echinometra van brunli, A. Ag. ,, vlridis, A. Ag. ,, rupicola, A. Ag. Lytechinus variegatus, A. Ac. Lytechinus semituberculatus, A. Ag. Tripneustes ventricosus, Ag. Trijmeustes depressns, A. Ag. Stolonoclypus ravenellii, A. Ag. Stolonoclypus rotandus, A. Ag. Mellita testudinata, Kl. Mellita longifissa, Mich.