CHAPTER 23 

 MODIFICATION OF SPECIES 



At many places in the preceding chapters tlie assumption has been 

 made that the kinds of hving things on the earth haxe clianged over 

 periods of time. In Chap. G possible ways of deriving multicellular 

 organisms from unicellular ones were postulated, in the belief that the 

 complex life of today could not always have existed. In describing the 

 varied breeding habits of animals (pages 185-186), it was assumed that 

 animals had evolved, but it was pointed out that the evolution of their 

 habits had not closely followed their structural evolution. In the classi- 

 fication of animals the basis of grouping is the supposed kinship of the 

 various species, due to descent from common ancestors and ascertained 

 from homology (pages 250^). The environmental relations of animals 

 were shown (page 283) to involve questions of evolution, since it was shown 

 that temperature could produce permanent modification of races. All 

 through the discussion of geographic distribution (Chap. 21) changes in 

 species were assumed to have occurred, in order to explain the position, 

 size, continuity, and proximity of ranges, and the differences between 

 southern and northern continents. And, finally, fossil animals (pages 

 331-348) were regarded as giving positive evidence not only of evolution 

 but of the direction which some evolutionary changes have taken. These 

 frequent references to evolution in advance of its separate discussion 

 indicate how intimately the idea of change of species is woven into the 

 entire fabric of biology. It would have been impossible to discuss these 

 phenomena adecjuately without relating them to evolution. Without 

 repetition of the facts and discussions already presented, it is left to 

 this chapter to summarize briefly with additions the reasons for believing 

 such changes to have occurred, and the methods by which they may have 

 been brought about. 



Evidences of Evolution. — One of the most compelling reasons for 

 assuming evolution is the existence of many similarities among species 

 of animals and plants. Some of these similarities have already been 

 detailed in the chapter on classification. To the homologies there 

 described may be added that shown by the membranous labyrinths of 

 the inner ears of vertebrate animals (Fig. 298). Each has a series of 

 three semicircular canals set in different planes and attached to a central 

 sac; but in each group of vertebrate animals there are characteristic 



349 



