198 Tin; EVOLUTION THEORY 



From the l»otanical point of view the assumption that flowers 

 and fio-\ver-visiting insects liave been adapted to each other by means 

 of processes of selection has lieen regarded as nntenable, because 

 every variation in the flower presupposes a corresponding one in the 

 insect. I should not have mentioned this objection had it not come 

 from such a famous naturalist as Niigeli, and if it were not both 

 interestino- and iiseful in our present discussion. Nageli maintained 

 that selection could not, for instance, have effected a lengthening of 

 the corolla-tube of a flower, because the proboscis of the insects must 

 have lengthened simultaneously with it. If the corolla-tube had 

 lengthened alone, without the proboscis of the butterfly being at 

 the same time elongated, the flower would no longer be fertilized 

 at all, and if the lengthening of the proboscis preceded that of the 

 corolla-tube it would have no value for the butterfly, and could 

 not therefore have been the object of a process of selection. 



This objection overlooks the facts that a species of plant and of 

 butterfly consists not of one individual but of thousands or millions, 

 and that these are not absolutely uniform, but in fact heterogeneous. 

 It is precisely in this that the struggle for existence consists — that 

 the individuals of every species difler from one another, and that 

 some are better, others less well constituted. The elimination of the 

 latter and the preferring of the former constitutes the process of 

 selection, which always secures the fitter by continually rejecting the 

 less fit. In the case we are considering, then, there would be, among* 

 the individuals of the plant-species concerned, flowers with a longer 

 and flowers with a shorter corolla-tube, and among the butterflies 

 some with a longer and some with a shorter proboscis. If among the 

 flowers the longer ones were more certain to be cross-fertilized than 

 the shorter ones, because hurtful visitors were better excluded, the 

 longer ones would produce more and better seeds, and would transmit 

 their character to more descendants ; and if, among the butterflies, 

 those with the longer proboscis had an advantage, because the nectar 

 in the longer tubes w^ould, so to speak, be reserved for them, and they 

 would thus be better nourished than those with the shorter proboscis, 

 the number of individuals with long proboscis must have increased from 

 generation to generation. Thus the length of the corolla-tube and 

 the length of the proboscis would go on increasing as long as there 

 was any advantage in it for the flower, and both parties must of 

 necessity have varied ^x(ri passu, since every lengthening of the 

 corolla was accompanied by a preferring of the longest proboscis 

 variation. The augmentation of the characters depended on, and 

 could only have depended on, a guiding of the variations in the 



