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PACIFIC SCIENCE, Vol. XXII, July 1968 
species at great distances from the shore. 
Nevertheless, he found that only 30 per cent 
are endemic. This can be compared with the 
approximately 80 per cent endemics which have 
been estimated for the entire Hawaiian flora 
(St. John, 1946; Hillebrand, 1888). A more 
recent estimate (Stone, 1967) is that specific 
endemism could amount to about 96 per cent. 
This bespeaks rather continuous arrival of strand 
species by long distance dispersal. The per- 
centage of dioecism in strand plant species from 
some low Pacific islands to the west of Hawaii 
was estimated by Long (1965, personal com- 
munication). Long concludes that less than 1.0 
per cent of the strand plant species with which 
he is familiar are dioecious. The evidence from 
McCaughey (1918) supports the concensus 
that most strand plants have reached the shores 
upon which they are established by means of 
long distance dispersal. Long’s estimates support 
the contention of Baker that long-distantly dis- 
persed species are self-compatible. 
In dealing with self-compatibility and dioe- 
cism it must, of course, be kept in mind that 
dioecism is not the only mechanism for produc- 
ing outcrossing. However, it is easier to recog- 
nize self-incompatibility at the level either of 
dioecism or heterostyly than at the chemical 
level of self-incompatibility. Just because a 
species does not show dioecism is no indication 
that it does not have other mechanisms pro- 
viding for outcrossing. Nevertheless, dioecism 
percentage does give an indication of percentage 
of outcrossing. Baker (1959<^:181) writes, 
". . . We might expect that the proportions of 
hermaphrodite species relative to those with 
separate staminate and pistillate flowers (par- 
ticularly the dioecious species) will give at least 
a very crude indication of the prevalence of 
outcrossing.” 
An assumption which is fundamental to 
Baker’s Law is that species that show a higher 
percentage of one breeding system under one 
set of conditions can change to another breeding 
system under a different set of conditions. 
Davis and Hey wood (1963), Grant (1958), 
and Fryxell (1957) have emphasized that the 
degree of outcrossing in the same species may 
vary under different conditions from as much 
as 50 per cent to as low as 2-3 per cent such as 
occurs in cotton. In support of this is the 
experimental work of Jones (1932, 1934) who 
was able to produce a dioecious strain of Zed 
mays from a monoecious strain. A dioecious 
species of Mussaenda (Rubiaceae) appears to 
have evolved from a heterostylous form. Orn- 
duff (1966) states that dioecism has probably 
arisen from heterostyly in the Menyanthaceae. 
Taylor’s (1954) observations on Coprosma 
pumila provide another example. 
There seems to be abundant support both 
theoretical and observational for the thesis so 
well expounded by Baker that the breeding 
system is of the utmost importance in successful 
long distance dispersal. There is a clear relation 
between self-compatibility and long-distantly 
dispersed species. This is made possible because 
breeding systems are dynamic. The breeding 
system can change in a given species when there 
is selection for self-compatibility, as, for ex- 
ample, in an area where only a few individuals 
of a given population are present. This seems to 
occur, both toward the margins of a popula- 
tion’s distribution and also in totally new areas 
which are attained by long distance dispersal. 
DIOECISM IN HAWAII 
Accepting the validity of Baker’s Law, the 
question arises of how to explain the high de- 
gree of dioecism in the Hawaiian flora. Dioecism 
is certainly an outcrossing breeding system, and 
the Hawaiian Islands are isolated. The origin 
of the flora can be explained in one or both of 
the two possible ways: either (1) there has in 
the past been closer connection with other land 
masses, or (2) the plants arrived by long dis- 
tance dispersal. Before attempting to show how 
Baker’s Law can be equated with the high de- 
gree of dioecism in the Hawaiian flora, let us 
see if there actually is a high degree of dioecism 
relative to a continental flora. 
Observations of botanists who have worked 
on continental floras and also that of Hawaii 
have led them to suspect that Hawaii has a 
disproportionate number of dioecious species 
(Carlquist, 1965, 1966^, 19 66b). Tabulations of 
the dioecious species for the Hawaiian flora were 
made using Hillebrand’s Flora of the Hawaiian 
Islands (1888), and for a continental flora 
using Munz’s Manual of Southern California 
Botany (1935). Admittedly the sample which 
