208 THE EVOLUTION THEORY 



In his important work, Mechanische-physiologische Theorie der 

 Ahstammungslehre, published in 1884, Nageli, as a convinced 

 opponent of the theory of selection, attempted an explanation. He 

 was quite aware that his assumption of an inward 'perfecting 

 principle' would not suffice to explain the mutual adaptations of 

 flowers and insects, and he refers the transformation of the first 

 inconspicuous blossoms into flowers to the mechanical stimulus which 

 the visiting insects exerted upon the parts of the blossom. By the 

 pressure of their footsteps, the pushing and probing with their 

 proboscis, they have, he says, transformed gradually, for instance, the 

 little covering leaves at the base of a pollen vessel into large flower 

 petals, caused the conversion of short flower-tubes into long ones, 

 and of the pollen, once dry and dusty, into the firmly adhesive 

 mass formed in the anther lobes of our modern flowers. The colour of 

 the flowers depends, according to him, upon the influence of light, which 

 certainly no more explains the yellow ring on a blue ground in the 

 forget-me-not than it does the many other nectar-guides which show 

 the insect the way to the honey. Nageli works with the Lamarckian 

 principle in the most daring way, and with the same naivete as 

 Lamarck himself in his time, that is, without offering any sort of 

 explanation as to how the minute impression made, say by the foot 

 or by the proboscis of an insect, upon a flower, is to be handed on to 

 the flowers of succeeding generations. He treats the unending chain 

 of generations as if it were a single individual, and operates with his 

 ' secular ' stimulus, and with ' weak stimuli, lasting through countless 

 generations,' as though they were a proved fact. But I have not 

 even touched upon the question as to whether these 'stimuli' could 

 produce the changes he ascribes to them, even if they were continually 

 affecting the flower. How the scale-like covering leaves of the 

 pollen vessels could become larger and petal-like through the treading 

 of an insect's foot is as difficult to see as why a honey-tube should 

 become longer because of the butterfly's honey-sucking : might it not 

 just as well become ivider, narrower, or even shorter ? I see no con- 

 vincing reason why it should become longer \ And even if it did 

 so, it would necessarily continue to lengthen as time went on, and 

 this is not the case, for we find corolla-tubes of all possible lengths, 

 but, it is to he noted, ahuays in harmony tuith the length of the 

 proboscis of the visiting insect. In a similar way Henslow has 

 recently attempted to refer the origin of flowers to the mechanical 

 stimulus exercised upon it by the visiting insects. ' An insect hanging 

 to the lower petal of a flower elongates the same by its weight, and 

 the lengthened petal is transmitted by heredity.' . . . ' The irritation 



