26 The Selection Theory 
that the long skeletal processes which grow out from these organisms 
have significance not merely as a supporting skeleton, but also as an 
extension of the superficial area, which increases the contact with 
the water-particles, and prevents the floating organisms from sinking. 
It has been established that the processes are considerably shorter 
in the colder layers of the ocean, and that they may be twelve times 
as long’ in the warmer layers, thus corresponding to the greater or 
smaller amount of friction which takes place in the denser and less 
dense layers of the water. 
The Peridineae of the warmer ocean layers have thus become 
long-rayed, those of the colder layers short-rayed, not through the 
direct effect of friction on the protoplasm, but through processes 
of selection, which favoured the longer rays in warm water, since 
they kept the organism afloat, while those with short rays sank 
and were eliminated. If we put the question as to selection-value 
in this case, and ask how great the variations in the length of 
processes must be in order to possess selection-value; what can we 
answer except that these variations must have been minimal, and 
yet sufficient to prevent too rapid sinking and consequent elimina- 
tion? Yet this very case would give the ideal opportunity for a 
mathematical calculation of the minimal selection-value, although 
of course it is not feasible from lack of data to carry out the actual 
calculation. 
But even in organisms of more than microscopic size there must 
frequently be minute, even microscopic differences which set going 
the process of selection, and regulate its progress to the highest 
possible perfection. 
Many tropical trees possess thick, leathery leaves, as a protection 
against the force of the tropical raindrops. The direct influence of 
the rain cannot be the cause of this power of resistance, for the 
leaves, while they were still thin, would simply have been torn to 
pieces. Their toughness must therefore be referred to selection, 
which would favour the trees with slightly thicker leaves, though 
we cannot calculate with any exactness how great the first stages 
of increase in thickness must have been. Our hypothesis receives 
further support from the fact that, in many such trees, the leaves . 
are drawn out into a beak-like prolongation (Stahl and Haberlandt) 
which facilitates the rapid falling off of the rain water, and also 
from the fact that the leaves, while they are still young, hang 
limply down in bunches which offer the least, possible resistance to 
the rain. Thus there are here three adaptations which can only be 
interpreted as due to selection. The initial stages of these adaptations 
must undoubtedly have had selection-value. 
1 Chun, Reise der Valdivia, Leipzig, 1904. 
