32 Evolution and Adaptation 
EVIDENCE IN FAVOR OF THE TRANSMUTATION THEORY 
EVIDENCE FROM CLASSIFICATION AND FROM COMPARATIVE 
ANATOMY 
It does not require any special study to see that there are 
certain groups of animals and of plants that are more like 
each other than they are like the members of any other group. 
It is obvious to every one that the group known as mammals. 
has a combination of characters not found in any other 
group; such, for instance, as a covering of hair, mammary 
glands that furnish milk to the young, and a number of other 
less distinctive features. These and other common character- 
istics lead us to put the mammals into a single class. The 
birds, again, have certain common characters such as feathers, 
a beak without teeth, the development of a shell around the 
egg, etc., and on account of these resemblances we put them 
into another class. Everywhere in the animal and plant 
kingdoms we find large groups of similar forms, such as the 
butterflies, the beetles, the annelidan worms, the corals, the 
snails, the starfishes, etc. 
Within each of these groups we find smaller groups, in 
each of which there are again forms more like each other 
than like those of other groups. We may call these smaller 
groups families. Within the families we find smaller groups, 
that are more like each other than like any other groups in 
the same family, and these we put into genera. Within the 
genus we find smaller groups following the same rule, and 
these are the species. Here we seem to have reached a limit 
in many cases, for we do not always find within the species 
groups of individuals more like each other than like other 
groups. Although we find certain differences between the 
individuals of a species, yet the differences are often incon- 
stant in the sense that amongst the descendants of any in- 
dividual there may appear any one of the other variations. 
If this were the whole truth, it would seem that we had here 
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