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 
