20 ZOOLOGICAL EXERCISES. 
by its relatives, will succeed better in life, and will leave 
more offspring, some of whom may possibly inherit these 
special properties in a more itensified degree, and may pass 
them on to their offspring still further developed. In this way 
varieties may beceme species. But natural selection alone 
is quite Inadequate to explain the origin of all species, and 
we cannot doubt but that it acts in conjunction with some 
unknown law that makes towards a pre-determined end. 
CLASSIFICATION. 
When we remember the enormous number of animals 
found on the earth, the necessity for a classification which 
will divide them into groups, and so enable us to condense 
our knowledge, is obvious. But the sole aim of classification 
is not convenience of reference; it has the higher aims of 
showing the relations between the different groups, and of 
tracing out the branches of the family tree which connect 
all animals together; in other words, of exhibiting the 
pedigree of all existing animals. As the final construction 
of this genealogical tree could only be the result of an 
almost complete knowledge of all animals, both living and 
extinct, it follows that we shall never have a perfect classi- 
fication, because we can never have a sufficiently complete 
knowledge of extinct animals; and it also follows that at 
present our classificatory tree is incomplete and provisional, 
and 1s constantly being changed as our knowledge increases. 
Animals are divided into sroups called sub-kingdoms 
(or phyla), classes, orders, families, genera, species, and 
varieties ; the first groups being larger than those that 
follow. All, however, are conventional or artificial, A 
genus is generally made when several species exist that 
exhibit some peculiarity; but genera, families, and even 
orders, may consist of but one species. Formerly each 
