IN THE THEORY OF NATURAL SELECTION. 305 



with short ones, the long- " tongues " from short ones. Undoubtedly 

 both have been developed at the same rate so that the length of 

 both sets of structures has always remained the same.' 



No objection can be raised against these statements, but Nageli 

 goes on to say : ' But how can such a process of development be 

 explained by the theory of natural selection, for at each stage in 

 the process the adaptation was invariably complete. The tube of 

 the corolla and the " tongue " must have reached, for instance, at 

 a certain time, a length of 5 or 10 mm. If now the tube of the 

 corolla became longer in some plants, such an aWferation would have 

 been disadvantageous because the insects would be no longer able to 

 obtain food from them, and would therefore visit flowers with 

 shorter tubes. Hence, according to the theory of natural selection, 

 the longer tubes ought to have disappeared. If on the other hand 

 the " tongue " became longer in some insects, such a change would be 

 superfluous and should have been given up, according to the same 

 theory, as unnecessary structural waste. The simultaneous change 

 in the two structures must, according to the theory of natural 

 selection, be due to the same principle as that by which Miinch- 

 hausen pulled himself out of a bog by means of his ow r n pig-tail.' 



But, according to the theory of natural selection, the case appears 

 in a very different light from that in which it is put by Nageli. 

 The flower and the insect do not compete for the greater length of 

 their respective organs : all through the gradual process, the flower 

 is the first to lengthen its corolla and the butterfly follows. Their 

 relation is not like that between a certain species of animal and 

 another which serves as its prey, where each strives to be the 

 quicker, so that the speed of both is increased to the greatest possible 

 extent in the course of generations. Nor do they stand in the 

 same relation as that obtaining between an. insectivorous bird and a 

 certain species of butterfly which forms its principal food ; in such 

 a case two totally different characters may be continually increased 

 up to their highest point, e.g. in the butterfly similarity to the 

 dead and fallen leaves among which it seeks protection when 

 pursued, in the bird keenness of sight. As long as the latter 

 quality is still capable of increase, so long will it still be advanta- 

 geous to any individual butterfly to resemble the leaf a little more 

 completely than other individuals of the same species ; for it will 

 thus be capable of escaping those birds which possess a rather 



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