Chap. VU.] THEORY OP NATURAL SELECTION. 31I 



With such structures as the above lamellEe of horn or 

 whalebone, habit or use can have done little or nothing, 

 as far as we can judge, towards their development. On 

 the other hand, the transportal of the lower eye of a 

 flat-fish to the upper side of the head, and the formation 

 of a prehensile tail, may be attributed almost wholly to 

 continued use, together with inheritance. With respect 

 to the mammae of the higher animals, the most probable 

 conjecture is that primordially the cutaneous glands 

 over the whole surface of a marsupial sack secreted a 

 nutritious fluid; and that these glands were improved 

 in function through natural selection, and concentrated 

 into a confined area, in which case they would have 

 formed a mamma. There is no more difficulty in under- 

 standing how the branched spines of some ancient Echi- 

 noderm, which served as a defence, became developed 

 through natural selection into tridactyle pedicellariae, 

 than in understanding the development of the pincers 

 of crustaceans, through slight, serviceable modifications 

 in the ultimate and penultimate segments of a limb, 

 which was at first used solely for locomotion. In the 

 avicularia and vibracula of the Polyzoa we have organs 

 widely different in appearance developed from the same 

 source; and with the vibracula we can understand how 

 the successive gradations might have been of service. 

 With the pollinia of orchids, the threads which origin- 

 ally served to tie together the pollen-grains, can be 

 traced cohering into caudicles; and the steps can like- 

 wise be followed by which viscid matter, such as that 

 secreted by the stigmas of ordinary flowers, and still 

 subserving nearly but not quite the same purpose, be- 

 came attached to the free ends of the caudicles; — all 

 these gradations being of manifest benefit to the plants 

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