THE ORIGIN OF ANIMAL SPECIES 459 



pairs of muscles, the spinal nerves and ganglia, the 

 intercostal arteries and veins. 



A comparison of such dissimilar organs as the wing 

 of the bat and the bird, the flipper of the seal, the pec- 

 toral fin of the fish, the hoof of the horse, and the hand 

 of man shows evidence of genetic relationship in that all 

 are constructed on one fundamental plan, which has 

 been modified to meet the needs of different environ- 

 ments. 



The testimony of rudimentary organs is also in favor 

 of the theory of descent with modification. The embryo 

 of the whalebone whale has teeth, but they never cut 

 the gum. Their presence is explainable only on the 

 hypothesis that this animal is a descendant of some form 

 that had functional teeth. Nearly half of the beetles 

 inhabiting the wind-swept island of Madeira have such 

 rudimentary wings that flight is impossible, though there 

 is no doubt that these insects were once capable of fly- 

 ing, since the nearest related species which live on the 

 mainland have fully developed wings. In this case, 

 inability to fly is a distinct advantage, because it renders 

 the insect less likely to be blown out to sea and drowned. 

 The presence of rudimentary and functionless eyes in 

 cave-inhabiting animals indicates descent from ancestors 

 having perfect visual organs. 



(3) Evidence from Embryology. Of the many im- 

 portant facts which this branch of science offers in 

 support of the theory of evolution, only a few can be 

 mentioned here. It has been learned that higher ani- 

 mals in the course of their embryonic development pass 

 through stages which are permanent conditions in lower 

 forms. Thus the bird and the mammal, though they 

 never possess gills in their adult life, have at an early 

 stage of existence a series of openings, gill slits, in the 

 side of the neck, corresponding to the gill openings of 



