Chap. vii. Theory of Natural Selection. 1 99 



the continued destruction of those which could not browse so high, 

 would have sufficed for the production of this remarkable quad- 

 ruped; but the prolonged use of all the parts together with inherit- 

 ance will have aided in an important manner in their co-ordination. 

 With the many insects which imitate various objects, there is no 

 improbability in the belief that an accidental resemblance to some 

 common object was in each case the foundation for the work of 

 natural selection, since perfected through the occasional preservation 

 of slight variations which made the 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 favourable variations, until the points were 

 •converted first 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 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 lamellae 

 are first used as teeth, then partly as teeth and partly as a sifting 

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



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

 bone, hahit 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 Echinoderm, 

 which served as a defence, became developed through natural selec- 

 tion into tridactyle pedicellariae, than in understanding the develop- 

 ment of the pincers of crustaceans, through slight, serviceable modi- 

 fications 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 



