THEORY OF NATURAL SELECTION. 221 



resemblance at all closer ; and this will have been carried on 

 as long as the insect continued to vary, and as long as a 

 more and more perfect resemblance led to its escape from 

 sharp-sighted enemies. In certain species of whales there is 

 a tendency to the formation of irregular little points of horn 

 on the palate ; and it seems to be quite within the scope of 

 natural selection to preserve all favorable variations, until the 

 points were converted, rirst into lamellated knobs or teeth like 

 those on the beak of a goose — then into short lamellae, 

 like those of the domestic ducks — and then into lamellae as 

 perfect as those of the snoveller-duck — and finally into the 

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

 whale. In the family 01 the ducks, the lamellae are first 

 used as teeth, then partly as teeth and partly as a silting 

 apparatus, and at last almost exclusively for this latter 

 purpose. 



With such structures as the above lamellae of horn or 

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

 far as we can judge, toward 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 prehen- 

 sile tail, may, be attributed almost wholly to continued use, 

 together with inheritance. Witn respect to the mamma? 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 tn rough natural selection, 

 and concentrated into a confined area, in which case they 

 would have formed a mamma. There is no more difficulty 

 in understanding how the branched spines of some ancient 

 Echinoderm, 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 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 vibracula 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 like- 

 wise be followed by which viscid matter, such as that secreted 

 by the stigmas of ordinary flowers, and still subserving 



