76 THE LESSON OF EVOLUTION 



to evolution is confirmed by the study of living 

 animals. 1. By what are called homologies in the 

 skeleton. That is, when the habits of a group of 

 animals change, the skeleton becomes modified to 

 the necessities of the change, not by the formation 

 of new bones instead of the old ones, but by the 

 modification of the old bones so as to enable them 

 to undertake new functions. The fore-limb has 

 changed into a flying organ in different ways in 

 birds, in bats, and in flying reptiles. 2. By the 

 development of the individual, in which we often 

 see that the living animal passes rapidly through 

 changes which were permanent in fossil forms. 

 This has been proved in the case of the shelly loop 

 which supports the arms in the Brachiopods ; in the 

 shells of Ammonites ; and in the skeletons of many 

 reptiles and mammals. I might give as an example 

 the tail in the bony fishes of the present day, which 

 is at first diphycercal, then becomes heterocercal, 

 and finally homocercal. Another example is the 

 Tuatara of New Zealand (Sphenodon punctatus) the 

 embryo of which passes through stages in which the 

 skeleton shews characters, some of which were per- 

 manent in the Stegocephalian Amphibians of the 

 Carboniferous, others in the early Khynchocepha- 

 lians of the Permian and Triassic periods. 3. The 

 geographical distribution of animals is explained by 

 the theory of descent. We find that not only are 

 the individuals of a species grouped together in a 

 specific area, but the species forming a genus are 

 also grouped together in a definite generic area. 

 This is not likely to have been the case if each 



