368 ANIMAL BIOLOGY 



perhaps may bring into line, all the data warrant the conclusion that 

 the chemical characteristics of the blood are almost as constant 

 as structural similarities of the blood vessels. Indeed, the inorganic 

 salts present in the various circulating fluids of animals correspond 

 in nature and relative amounts to what we have good reason to 

 believe was the composition of the ocean some hundred million 

 years ago. So in evolutionary terms, a common property has per- 

 sisted in the bloods of animals throughout the ages which have 

 elapsed during their evolution from a common ancestor: of all 

 the systems, the blood perhaps is the most conservative in retain- 

 ing its ancestral condition. Blood relationship is a fact. 



6. Distribution 



Everyone recognizes that there are wide variations in the fauna 

 and flora of different parts of the Earth. There is a characteristic 

 life on mountain, plain, and seashore, and in the sea - - as well as 

 in pond and puddle — and also in arctic, temperate, and tropical 

 regions. But the problem of animal and plant distribution is by 

 no means so simple as this statement might seem to imply, because 

 the study involves the investigation of both the relations of the 

 various organisms to the general environing conditions, and the 

 interrelations of the species with each other. It forms a part of the 

 sciences of plant and animal ecology. (Figs. 220, 236.) 



Confining attention merely to the geographical distribution of 

 animals - which forms the science of zoogeography - - let us take 

 a couple of clear-cut examples and see whether or no evolution 

 offers a reasonable explanation of the facts. 



A characteristic genus of Mammals, known as the Tapirs, is 

 represented to-day by distinct species in two widely separated 

 regions: Central and South America and southern Asia and ad- 

 jacent islands. But distribution in the past proves to be the key 

 to the present distribution. Paleontological studies show that in 

 the Pliocene epoch Tapirs were distributed over nearly all of North 

 America, Europe, and northern Asia, and thereafter gradually 

 became extinct so that by the close of the Pleistocene epoch the 

 remnants were distributed as we find them to-day. In brief, the 

 present discontinuous distribution represents the remnants of a 

 world-wide Tapir population, and the differences between the 

 existing species are such as one might expect to find among the 

 members of a genus long isolated in different environments by 



