4B8 ROSS — -ON EVOLUTION. 



of America. Species of the same Family found in localities long 

 isolated from the chief existing centre of differentiation, for that 

 family are usually smaller, less vigorous, and less fully differenti- 

 ated than the others, resembling the immature forms of the more 

 differentiated Species. 



A most interesting and suggestive fact in the distribution of 

 Organic Types is the existence in Regions more or less recently 

 isolated from each other, of representative Families, Sub-families, 

 Genera, Sub-genera, Species, or Sub-species, according apparently 

 to the length of time the isolation has existed, and to the rapidity 

 with which differentiation takes place in the particular Group 

 selected for comparison, and that in lands long isolated from each 

 other the Organic Types are very different, however similar may be 

 the climatic and other conditions. In Europe and North America, 

 which have probably been separate since the Miocene Epoch, many 

 Genera exist having a certain number of Species in the one corres- 

 ponding to a certain number in the other, each to each. 



I quote from Prof. Wyville Thompson : "On either side of 

 the Isthmus of Panama the Echinoderm order Echinidia, 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 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 

 arranoe a few of the most marked of these from the Caribbean and 

 Panamic sides of the Isthmus in parallel Columns. 



EASTERN FAUNA. WESTERN FAUNA. 



Cidaris annulata^ Gray. Cidaris thouarisii, Val. 



Diadema antillarum, Phil. Diadema mexicanum, A. Ag, 

 Echinocidaris punctulata^ Echinocidaris 8tellata, Ag. 



Desml. 

 Echinometra michelmi, Des. Echinometra van hrunti^ 



A. Ag. 



Echinometra viridis^ A. Ag. Echinometra rupicola, A. Ag. 



Lytechinus vai^iegatus, A. Ag. Lytechinus semituberculatus, 



A. Ag. 



