IX, G, 1 Merrill: Plants of Guam 21 
Guam, have been introduced into the island purposely or inad- 
vertently by man. Excluding these 314 species from considera- 
tion, the endemism is then about 27 per cent. This percentage, 
however, is very small when compared with that of the Philip- 
pines, where, including all introduced species, the endemism is 
about 40 per cent; the Philippines, however, are very much closer 
to other large land masses than is Guam, which, while in part 
explaining the richness of the Philippine flora, does not explain 
its high percentage of endemism in comparison to the low en- 
demism of Guam. 
From a geological standpoint Guam is undoubtedly recent, 
a claim that is substantiated by its very poor indigenous flora, 
but 225 species being known from Guam that can be considered 
truly indigenous, and by its very low percentage of endemism, 
11 per cent, if we take into consideration the introduced species, 
_ and but 27 per cent, if we consider only the indigenous species. 
It has been seen that the flora of Guam is a relatively poor 
one, at least so far as we at present know it, especially for an 
island of its size located in the rainy tropics. Its low percentage 
of endemism is especially noticeable in view of its rather isolated 
location. Both of these features are readily explained, however, 
on the recent origin hypothesis. 
Perhaps the most interesting feature of the Guam flora is the 
introduced element, and as this was not discussed in detail by 
Safford, some attempt will here be made to analyze the con- 
stituents of the introduced flora, the origin of the species, and 
the time and method of their introduction. As Guam was a 
regular stopping place for the Acapulco-Manila galleons, for a 
period of nearly three hundred years, a study of the introduced 
element is of especial interest in view of the fact that Guam has, 
without doubt, served as a center of distribution for American 
weeds to the other islands of Micronesia and Polynesia, even 
as the Philippines served the same end for parts of the Malayan 
region and of tropical Asia. A discussion of the vegetation in 
relation to the weed flora may explain certain problems regard- 
ing pantropic weeds and their origin, and especially in regard to 
the occurrence in Polynesia of certain weeds that are generally 
considered to have been of American origin, but which were 
found in Polynesia in the last half of the eighteenth century, 
by the first botanists who visited this part of the world. 
The introduced element in the Guam flora may be divided into 
four groups or time periods. The first of these is the prehistoric 
period, of many centuries duration, in which Guam was peopled 
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