BARBOUR AND NOBLE: LIZARDS OF THE GENUS AMEIVA. 419 
p. 562) and Barbour (Mem. M. C. Z., 1914, 54, p. 326) have suggested 
that the Antillean Ameivas were derived from a centre of dispersal 
in northeastern South America, and that they have spread thence 
northward over a continuous land area to the Greater Antilles. Pro- 
ceeding northward along the islands we find species which show a 
gradual transition in morphological characters, and there is no obvious 
break in the series, except where the evidence is wanting, as for ex- 
’ ample where the species on Dominica seems very different from that 
of St. Vincent, we must remember that the form which formerly 
inhabited Martinique is undescribed zodlogically and is probably now 
extirpated by the mongoose. This gradual transition, as we have 
said, points to a land migration and not to distribution by flotation. 
The latter means would not account for the presence of the genus 
upon so many islands, without presupposing an enormous amount of 
rafting. Such a constant flotation would have kept new immigrants 
coming to the islands already populated, as well as to those as yet with- 
out Ameivas, and would surely have tended to keep the whole Antil- 
lean group of individuals more homogeneous than they are. There 
is no real reason for supposing that there was more carriage in the 
past than at present. Then the derivation would probably have been 
from several stocks, whereas the Lesser Antillean Ameivas are all 
derived from the Ameiva ameiva stock, the Antillean and mainland 
races having probably had a common origin from an ancestral wide- 
spread stock which became differentiated as the stations occupied 
became separated. The comparatively fixed characters observed 
among the individuals of the island races stand at sharp contrast to 
the great variability of the same characters in the mainland races, 
and this points to a long complete isolation. Interchange of indi- 
viduals between the islands is unthinkable on any basis, as their 
physical geographic characters make the setting free of rafts impossible. 
By the flotsam theory individuals must have reached all islands by 
rafts directly from mainland rivers. 
Gadow (P. Z. S., 1906, p. 277-375) has shown that the closely 
related genus Cnemidophorus is composed of species having remark- 
ably variable characters and that it is necessary to consider the sum 
of the distinguishing features when comparing two forms. Similarly 
in Ameiva too great stress cannot be laid upon a single character 
within a species, especially upon the mainland. This variability may 
make two species, probably but distantly related, appear closely 
similar.. Some of these curiously close resemblances between widely 
separated forms may be mentioned, as they are interesting from an 
