268 
tral Pacific counterparts appear to be of a ran- 
dom nature. Within any family of fishes rep- 
resented in Hawaii, the endemic forms are 
often the most abundant. 
In striking contrast with the Hawaiian 
marine biota, the native terrestrial biota is 
highly disharmonic or unbalanced. Great 
groups of animals, e.g., the amphibians, were 
completely unrepresented, whereas others, e.g., 
the land snails and drepaniids, proliferated 
greatly. Not only new Hawaiian species, but 
also new genera and families evolved. Among 
the achatinellid land snails, the genus Achati- 
nella is restricted to the island of Oahu, but 
some 100 allopatric forms have been described. 
No relationship between the peculiarities of 
these forms and the environment they inhabit 
has ever been demonstrated. The drepaniid 
finches seem to have evolved in quite a different 
way. They inhabited all of the major islands 
of the Hawaiian chain and some of the smaller 
islands as well. The most notable differentation 
within the group is in beak shape, which is 
associated with feeding habits (Baldwin, 
1953). Several different drepaniids were often 
sympatric. 
One of the main differences between the 
terrestrial and marine environments in Hawaii 
is in the amount of change caused by man. 
The terrestrial environment has been largely 
transformed, in part directly by man via agri- 
culture, etc., but perhaps more by the indirect 
effect of animals and plants which man has 
introduced, intentionally or unintentionally. 
Many of these introductions have now replaced 
or are replacing the native biota and are directly 
or indirectly responsible for the restriction or 
extinction of native forms. 
With this brief background, the question of 
evolutionary processes will be discussed. 
Basic to the evolution (or lack of it) that 
will occur on any island is the matter of which 
organisms are there and which are not. To 
exist, an organism must first arrive, and it then 
must find an environment in which it can sur- 
vive and reproduce. Both of these aspects de- 
pend in part on the isolation of the island — 
not isolation in terms of geographical-physical 
barriers alone, but in terms of these in relation 
to the ability of the organism to cross them. 
The day when isolation could be considered 
PACIFIC SCIENCE, Vol. XXII, April 1968 
a causal factor has long since passed. However, 
that it is a powerful controlling factor is gen- 
erally recognized. This control acts in two re- 
lated ways. First, it determines which organisms 
will get to an island and which will not. The 
selectivity of this filtering factor will increase 
with increasing isolation and hence will deter- 
mine in part the extent to which the island 
biota resembles its parental biota. The greater 
the difference between these two biotas, the 
greater will be the change in biological selec- 
tion pressures on any organism arriving on the 
island. This point will be discussed later. 
Second, any species that establishes itself on 
an island should, at least for a while, be pre- 
served from contamination by gene flow from 
the parental population. If the recent introduc- 
tion to Hawaii of numerous species (e.g., the 
Marquesan sardine, the African snail, the 
garden spider, etc.) is any criterion, the initial 
immigrants can build up a population of mil- 
lions of individuals in a few years. Beyond this 
point contamination from gene flow from a 
few subsequent immigrants will probably have 
little effect (Gosline, 1958). There are, how- 
ever, certain important exceptions to this state- 
ment. If, in the process of building up a popu- 
lation from initial immigrants, the population 
becomes debilitated in some way or loses its 
ability to cope with diseases or parasites which 
later immigrants may bring with them, then 
subsequent immigration may matter a great 
deal. 
Of factors actually causing insular evolution 
only two will be considered. One is natural 
selection, and the other the series of features 
associated with small population size. 
It is generally agreed that differentation pro- 
ceeds more rapidly in animal populations on 
small islands than on large ones. The question 
is: to what extent is this caused by differences 
in the selective forces on small islands, and to 
what extent to factors associated directly with 
small population sizes. A rather large body of 
data suggests that many of the peculiarities of 
small-island forms are not directly selected by 
the enviroment. Two examples will suffice. 
Dowdeswell and Ford (1953 and elsewhere) 
have shown that on the larger islands of the 
Scilly group the spotting on the wing of the 
butterfly Maniola jurtina remains about as it 
