88 The Insides and the Wor\ings of Living Things 



of species, show numerous examples of extreme specialization 

 (Fig. 2i). Plants also present to us graded series from the 

 simplest types, in which each cell does everything that pertains 

 to being alive, to the complex ferns and seed plants, in which 

 there are many kinds of organs and many kinds of tissues 

 adapted to the various vital processes and to specialized condi- 

 tions of living. Finally, within each type, or even smaller sub- 

 division, it is possible to find gradations of organization 

 from the more general to the more specialized. These facts 

 point to evolution where we can apply to them the test of 

 order in time. As we saw (page 35), the more generalized 

 forms of any series appear in older rocks, the more specialized 

 forms appear in more recent rocks. 



Homology 



Within the group of animals that belong to the same type 

 or plan of structure there is a remarkable adherence to the 

 pattern, even where the conditions of life are radically dif- 

 ferent, and where a direct adaptation of organs to particular 

 needs would seem to be called for — if we assume for each 

 species a special act of creation. This is illustrated by the 

 facts assembled under the so-called principle of homology. 

 An extreme example of homologous organs that behave dif- 

 ferently in two different species is furnished by contrasting 

 a mammal like the whale, which lives in the water, and a 

 mammal like the cat, which lives on land. The striking 

 difference in outward appearance, in the form of the limbs, 

 in the habits of locomotion and feeding, and in hundreds of 

 other details go hand in hand with equally numerous simi- 

 larities in details of structure. 



Arms and Legs 



There are five distinct classes of backboned animals, 

 besides a number of species that are not so easily classified. 

 These five principal classes are the fishes, the amphibians 



