1 37 
The Flora of the Galapagos Islands. 
species of Acalypha , &c. In many instances these forms are confined 
to a single island, and in the majority of cases each form is more 
closely related to those of the other islands than to any continental 
ally. On the theory of origin by subsidence, these harmonic relations 
ar$ readily explained. If the islands were once united and then 
separated by subsidence, the remnants of their common flora and 
fauna, persisting upon the different islands, would have diverged not 
only from the continental types but from each other. From the 
depth of ocean between the archipelago and the mainland it would 
naturally be inferred that the islands were cut off from the continent 
before they were divided from each other, and this will explain the 
greater difference between the iorms and the continental type than 
between the forms themselves. 
Dr. Robinson, however, suggests an explanation which will 
accord with the theory of emergence. 
If we assume (i) that seed-transference between the mainland 
and the islands or between the islands themselves, does not occur in 
the case of particular plants oftener, on the average, than once in 
several years, and (2) that plants have multiplied on the islands as 
rapidly as they have frequently been observed to multiply elsewhere, 
it becomes as easy to account for the existence of a harmonic flora 
on islands of emergence as on islands of subsidence. 
On these premises an insular form having started its divergence 
from the continental type, would be likely to become more and more 
differentiated, and, owing to its existence in large numbers, would not 
be much affected by the occasional arrival of isolated and scattered 
seeds from the continent. Similarly if seeds of the new form were 
carried to other islands of the archipelago, lurther specialized races 
might arise bearing much the same relation to the original insular 
form as that did to its continental progenitor; and these secondary 
forms would be similarly as little affected by subsequent rare and 
isolated seed-arivals. Thus unchecked, the races might develop into 
more and more highly differentiated forms and varieties, and ultimately 
into well-marked species characteristic of particular islands. 
That this assumed infrequency of successful seed-transference does 
obtain in the case of the Galapagos, is clearly shown by the existing 
diversity in the floras of the different islands—a condition which could 
not continue if seed-transference were very common. The infrequency 
may be explained by the arid and sterile shores, which offer to most 
seeds washed ashore by oceanic currents a very poor chance of 
successful germination. 
Finally, Dr. Robinson draws special attention to the relation 
which the proximity of the different islands bears to likeness in their 
florulae. Although the islands are very different in altitude, 
climate and fertility, the diversity in their vegetation is greater than 
we should expect. The common element between any two rarely 
exceeds 75 per cent, and is often less than 50 per cent. Moreover 
these differences stand for the most part in no ielation to the distance 
of the islands from each other, or to the depths of the intervening 
channels. The florulae of Albemarle and Charles, islands at opposite 
ends of the archipelago, are more alike than either is to that of 
intervening islands, and cases in which proximity between two islands 
is associated with marked floral similarity are quite the exception. 
Such anomalies must find their explanation in peculiarities of 
