34 Galls, Gall'inakerSy and Cuckoo Flies. [Sess. 



of a gall-making insect appears, therefore, to exert an influence 

 on the development of growing tissue somewhat analogous to 

 that of the pollen-tube on the walls of the ovary, inducing 

 changes whereby the pistil or ovary is transformed into the 

 fruit. The analogy is strengthened by the fact that it has 

 been found possible to deceive a flower, so to speak, by arti- 

 ficial means. By irritating the stigma of the orchid Miltonia 

 Kusselliana, or by placing small pieces of grit upon it, the 

 ovaries have been made to swell just as though the flower had 

 been properly fertilised with pollen : the seeds in this case, 

 however, are destitute of an embryo and incapable of germin- 

 ating. In like manner the unfertilised eggs of the sea-urchin 

 have been stimulated into development by means of saline 

 solutions. 



Special interest attaches to the explanation which refers the 

 formation of galls to the irritation caused by larvae, since this 

 principle is extended by Henslow to account for all the modi- 

 fications which flowers have undergone whereby they have 

 become adapted to cross-fertilisation by insects. The brilliant 

 colours, the curious shapes, the perfumes and nectar of flowers, 

 have reference to the visits of insects. These remarkable 

 adaptations in flowers Darwin attributed to natural selection 

 operating on accidental spontaneous variations, tending to 

 secure the advantages of cross -fertilisation. According to 

 Henslow, the adaptation of flowers to insects owe their origin 

 to the irritation set up by insects visiting the blossoms. The 

 stimulus so applied, by inducing an increased flow of sap to 

 certain parts, and giving rise to hypertrophy or overgrowth in 

 them and to atrophy in other parts, would, Henslow supposes, 

 initiate a series of changes by which in time a flower might 

 become specialised to a particular kind of insect. On this 

 view flowers have been moulded and fashioned by a process 

 very similar to that by which galls are produced. In support 

 of his view Henslow appeals to the obvious effects of strains 

 and stresses on growth, as seen in the bulging of the pear on 

 one side to resist the strain of gravity, and in the curvature of 

 the corolla of the dead-nettle and similar flowers. 



The curious relationships long known to subsist between 

 figs and certain species of Cynips have significance in this 

 connection. From time immemorial the Greeks and Italians 



