
IN NATURAL HISTORY 9 
Owing to the various facts just mentioned it is impossible to arrange 
animals in a straight line from lowest to highest; each animal is not 
oaly related to those before and behind, but to those on either side of it, 
and if one group of animals is compared with another it will be found 
that the lower members of one will be decidedly lower or simpler in 
structure than the highest members of the preceding group. The rela- 
tions of animals to each other are often expressed in the form of a tree. 
The trunk represents the common origin of animals, the branches the 
great groups (see Classification), and the tips of the twigs individuals. 
Only it must be remembered that in the tree of life as we now see it 
many of the branches are lacking. 
The questions of evolution and of the animals that formerly existed 
are directly connected with the problem of distribution. 
GEOGRAPHICAL DISTRIBUTION 
Animals are not spread indiscriminately over the earth, but certain’ 
kiads or species are found in particular regions, some being confined to 
comparatively small areas, while others are widely distributed. Llamas 
are found only in South America; one kind of elephant occurs in Africa, 
another in parts of Asia; the moose, represented by three closely re- 
lated species, circles the entire northern hemisphere, while the orang- 
utan is restricted to a small part of Borneo, a second species being 
limited to a still smaller portion of Sumatra. This distribution is termed 
Geographical Distribution, and the branch of geography devoted to 
it—zoological, or more briefly zodgeography, the corresponding study of 
plant distribution being phytogeography. The problems of zodgeog- 
raphy are very complicated; for many causes have brought about the 
dispersal of animals, or caused their restriction to certain regions. 
The means by which animals that walk, swim, or fly have been 
distributed are apparent, while those less able to get about in the world 
are subject to winds, rivers, currents, floods, and accidental transporta- 
tion by other animals. The distribution of some takes place while they 
are still in the egg, or very small: for many animals, like the oyster, 
which are rooted to one spot when old, are free to wander while young. 
This may bring about results that at first sight seem contradictory, 
creatures whose powers of locomotion are small having a wider range than 
some well able to travel about. Such cases may sometimes be explained 
by the fact that the more active animals are less subject than the others 
to accidental dispersion, and are not swept away from places where food 
is abundant and enemies few. There are, however, many instances 

