4i6 SCIENCE PROGRESS. 



be of the same general service to metabolism as in the 

 cases already cited. Whether, in the case of extrafloral 

 nectaries, the red may be regarded as a signal to ants, 

 as Schimper (31) has suggested, requires further investi- 

 gation. 



Perhaps a fair picture of how anthocyan may act in 

 such cases as those just mentioned may be sketched as 

 follows. In organs of plants of high metabolic activity 

 tannins are of frequent occurrence. Tannins readily give 

 rise to the colour-bodies whose thermal properties are such 

 as may be conceived to be useful to the plant. External 

 conditions such as intense light and low temperature are 

 sufficient to set up the slight modification in direction of the 

 metabolism of the tannins resulting in the formation of the 

 colour-bodies. The principle of selection leads us to assume 

 that tJiCTefore the anthocyan is useful : whereas if we 

 followed the principle of experiment a whole series of 

 researches would be required to establish the final pro- 

 position : as, for example, what exactly is the normal course 

 of metabolism, how far can external conditions modify it 

 and how is the plant affected when, by appropriate means, 

 we prevent the formation of the anthocyan ? In short : the 

 explanation of an adaptation may be brilliant but cannot be 

 sound since it is based on only one of the two scientific 

 methods, deduction and induction. Hence to complete the 

 fair picture just sketched we must add that experiments, 

 to render it probable that the general mode of action of 

 anthocyan consists in increasing metabolism, are still to be 

 made. 



Stahl applies his discovery of the considerable heat 

 absorption by anthocyan to the case of anemophilous 

 flowers in which red is the strikingly predominant colour, so 

 much so, indeed, that it has led MacLeod (32) to express a 

 doubt as to whether colours are indeed special adaptations to 

 insect fertilisation. 



The wind-fertilised flowers in question are those of 

 many dicotyledonous trees such as the poplar, ash, elm, 

 alder and hazel, and of shrubby plants as Riuncx and 

 Poterium. In such plants, flowering during the uncertain 



