26 The Selection Theory 



that the long skeletal processes which gi-ow 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 suflicient 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 diflerences which set going 

 the process of selection, and regulate its progress to the highest 

 possible perfection. 



jNlany 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 M'ith 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 oft' of the rain water, and also 

 from the fact tliat 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. 



* Cbun, Reise der Valdivia, Leipzig, 1904. 



