Such
certainly do exist. They are fertile together, and yet each maintains
itself tolerably distinct. How can this be, if there is no
disinclination to crossing? My belief certainly is that number of
offspring is not so important an element in keeping up population of a
species as supply of food and other favourable conditions, because the
numbers of a species constantly vary greatly in different parts of its
area, whereas the average number of offspring is not a very variable
element.
However, I will say no more but leave the problem as insoluble, only
fearing that it will become a formidable weapon in the hands of the
enemies of Natural Selection.
While writing a few pages on the northern alpine forms of plants on the
Java mountains I wanted a few cases to refer to like Teneriffe, where
there are no _northern_ forms, and scarcely any alpine. I expected the
volcanoes of Hawaii would be a good case, and asked Dr. Seeman about
them. It seems a man has lately published a list of Hawaiian plants, and
the mountains swarm with European alpine genera and some species![68] Is
not this most extraordinary and a puzzler? They are, I believe, truly
oceanic islands in the absence of mammals and the extreme poverty of
birds and insects, and they are within the tropics.
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