THE FACTORS OF ATROPHY 255 



2. Sociology. With societies this elimination of 

 useless structures is effected much more easily than 

 with organisms for several reasons : 



In biology a special factor, heredity, gives to 

 specific characters a force which does not exist in 

 the same degree with social institutions. Now 

 functional organs common to a whole line of 

 descent are not easily effected by the influence 

 of individual surroundings. Further, the trans- 

 domesticated condition man provides food and the colour being 

 unimportant all variations may survive. 



(2) Loss of spines in plants on oceanic islands. It is well known 

 that the presence of spines protects plants from the ravages of 

 herbivorous animals, particularly mammals. But in oceanic islands 

 bats are generally the only mammalian inhabitants, and so, accord- 

 ing to "Wallace (Darwinism), there are no spiny plants in the 

 indigenous flora of St Helena. The much richer flora of the 

 Hawaian islands includes only a very few prickly plants. All 

 the endemic genera are unarmed, as also are most of the endemic 

 species of other genera ; even genera like Xanthoxylum, Acacia, 

 Xylosoma, Lycium, and Solanum, which are so frequently armed 

 in other countries, are there represented by unarmed species. 

 The two species of Rubus bear prickles reduced to the merest 

 points and the two palms are devoid of spines. How is the 

 absence of spines to be explained in these plants ? The plants 

 have been derived from the mainland, the seeds being brought by 

 the wind, by currents, or by birds, and having found soil have 

 germinated. In their new country they are not attacked by 

 herbivorous animals, and it is immaterial to them whether or no 

 they bear spines. The individuals badly armed are at no dis- 

 advantage compared with those possessing the normal armature ; 

 on the other hand, they have the advantage of being without 

 useless organs to support. Spines, in consequence, gradually 

 disappear. 



This struggle for existence may cause the disappearance of some 

 organisms themselves, and not only the atrophy of parts of them. 





