262 ORIGIN OF LIFE IN AMERICA 



not half-a-dozen species of land mammals, while Ireland pos- 

 sesses nearly three times that number. There are only two 

 ways in which we can account for this great deficiency in the 

 higher animal life on the Antilles. We may suppose that 

 either the islands have not been connected with the mainland 

 since early Tertiary times, or if they have, that their fauna 

 was largely destroyed since their isolation. In the latter 

 case the apparent poverty of the fauna might be due to great 

 destruction of animal life during a submergence of the land, 

 and the consequent reduction of the habitable area. On the 

 other hand, we should expect the relict fauna of the islands to 

 exhibit marked affinities with that of the two great continents 

 lying to the north and south respectively. However, as I 

 have mentioned, the fauna on the whole is essentially dis- 

 similar from that of North and South America. The problem 

 of the origin of the West Indian fauna, therefore, like that of 

 Centra] America, is much more complex than it would at first 

 sight appear. I alluded to the apparent poverty of the fauna 

 because, although it does appear very poor in the higher groups 

 some of the lower forms of animals are represented by a large 

 number of species. The land-snails, in fact, are extremely 

 varied in character, and the abundance of species is one of the 

 most remarkable features of the West Indian fauna. A study 

 of their distribution and their relationship will probably 

 give us a better insight into the origin of the fauna as a whole 

 than the higher vertebrates, which are so poorly represented 

 on the islands. 



Before describing the molluscan fauna of the Antilles, a 

 few preliminary remarks on the islands may not be out of 

 place. The islands as a whole form a natural breakwater or 

 barrier between the Atlantic Ocean on the one hand, and the 

 Gulf of Mexico and the Caribbean Sea on the other, which 

 lie on the opposite side of it (Fig. 13). A mere glance at the 

 map is sufficient to show that we have to deal with two distinct 

 sets of islands. A series of large ones belong together, viz., 

 Cuba, Haiti or San Domingo, Jamaica and Portorico, with 

 mountain crests running in an east-westward direction, while 

 a chain of the much smaller, Anguilla, Guadeloupe, Dominica, 

 Martinique, St. Vincent, Barbados and others, placed in north 

 and southward position, constitute quite an independent 



