148 THE LESSON OF EVOLUTION 



(Nesonetta), and of the flightless cormorant of the 

 Galapagos Islands ( ), are good examples. 



For flight could not have been disadvantageous to 

 water-birds, as it might be to land-birds on small 

 islands. The dwindling away of internal parts, such 

 as the pelvic-girdle in whales, sea-cows and snakes ; 

 the shoulder-girdle in some Moas, and the optic 

 nerve in blind animals, cannot be due to natural 

 selection. In the petrels the hind toe is reduced to a 

 claw, which is quite functionless, and it is difficult 

 to see what disadvantage a hind toe could have been 

 to these birds. But the petrels are an old group. 

 Bones have been found in beds of miocene age, 

 which shew that stormy-petrels, shearwaters, and 

 albatrosses all lived at that time; and, as the hind 

 toe is vestigial in all these groups, it probably be- 

 came so before they were differentiated. And we 

 cannot say what the habits of the birds may have 

 been so long ago. 



Attempts have been made to explain these cases 

 of degeneration by the so-called " principle of 

 economy of growth." I have, however, already 

 shewn that this principle does not rest on any evi- 

 dence ; and it is very unlikely that useless organs 

 should not receive sufficient nourishment to enable 

 them to develop properly ; for even in times of dearth 

 of food all organs suffer equally. But we have in 

 Bering's theory a simple explanation, viz., that 

 disused parts are gradually lost through forgetful- 

 ness. The organs, or cells, no longer remember 

 how to reproduce the long disused parts : the 

 stimulus dies away and they remain undeveloped. 



