GENERAL ZOOLOGICAL FACTS AND THEORIES 291 



(2) Evolution 



INTRODUCTION. In the list of phyla of the animal kingdom on 

 pages 5 to 6, the approximate number of described species be- 

 longing to each is given. The sum total of all these is 265,050 

 of living and 39,905 of fossil forms. 1 The origin of these different 

 species may be accounted for in three ways: (i) by special creation 

 (see p. 8), (2) by spontaneous generation (see p. 8), or (3) by 

 evolution. The first two theories are no longer considered seri- 

 ously by biologists; some of the evidence for evolution will be 

 found in the following pages. We are not concerned here with 

 the question of the origin of life; but shall try to show that spe- 

 cies have evolved from one another or have descended from 

 common ancestors. 



THE ARGUMENTS FOR EVOLUTION. Evidence may be de- 

 rived from the study of comparative anatomy, embryology, 

 paleontology, and geographical distribution to prove that organic 

 evolution has taken place. 



(a) Comparative Anatomy. Homologous structures, i.e. 

 structures that are anatomically similar, point to a common an- 

 cestry for the species possessing them. The fore limbs of the bird, 

 dog, man, and bat (Fig. 158) possess the same fundamental 

 structure, though each is modified for particular functions. Their 

 similarity is explained by the fact that the animals bearing them 

 have all descended from a common ancestor; their dissimilarity 

 by the fact that they are used for different purposes. Vestigial 

 organs, such as the muscles of the ear, and the appendix of man, 

 are structures which are of no use to him now, but were functional 

 in his ancestors. They indicate to us some of the characteristics 

 of these ancestors (256). 



(b) Embryology. This subject may be dismissed by referring 

 the reader to the discussion of the law of biogenesis on page 228. 



1 This is a very conservative estimate and is undoubtedly a lesser number 

 than have really been described. The number is increasing every year, since 

 new species are being described almost daily. 



