256 ORIGIN OF SPECIES 



like those of the domestic di'cks — and then into lamellae, as 

 perfect as those of the shoveller-duck, — and finally into the 

 gigantic plates of baleen, as in the mouth of the Greenland 

 whale. In the family of the ducks, the lamellse are first used 

 as teeth, then partly as teeth and partly as a sifting ap- 

 paratus, and at last almost exclusively for this latter purpose. 



With such structures as the above lamellae of horn or whale- 

 bone, 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 concen- 

 trated 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 Echino- 

 derm, which served as a defence, became developed through 

 natural selection into tridactyle pedicellariae, than in under- 

 standing 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 de- 

 veloped from the same source ; and with the virbracula we 

 can understand how the successive gradations might have 

 been of service. With the pollinia of orchids, the threads 

 which originally served to tie together the pollen-grains, can 

 be traced cohering into caudicles; and the steps can likewise 

 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, became attached to the free 

 ends of the caudicles; — all these gradations being of mani- 

 fest benefit to the plants in question. With respect to climb- 

 ing plants, I need not repeat what has been so lately said. 



It has often been asked, if natural selection be so potent, 

 why has not this or that structure been gained by certain 



