2 Evolution and Adaptation 
similar to, but of course much smaller than, that made by the 
mole. In both of these cases the adaptation is the more 
obvious, because, while the leg of the mole is formed on the 
same general plan as that of other vertebrates, and the leg of 
the mole-cricket has the same fundamental structure as that 
of other insects, yet in both cases the details of structure and 
the general proportions have been so altered, that the leg is 
fitted for entirely different purposes from that to which the 
legs of other vertebrates and of other insects are put. The 
wing of the bat is another excellent case of a special adap- 
tation. It is a modified fore-limb having a strong membrane 
stretched between the fingers, which are greatly elongated. 
Here we find a structure, which in other mammals is used 
as an organ for supporting the body, and for progression on 
the ground, changed into one for flying in the air. 
The tails of mammals show a number of different adapta- 
tions. The tail is prehensile in some of the monkeys; and not 
only can the monkey direct its tail toward a branch in order to 
grasp it, but the tail can be wrapped around the branch and 
hold on so firmly that the monkey can swing freely, hang- 
ing by its tail alone. The animal has thus a sort of fifth 
hand, one as it were in the middle line of the body, which can 
be used as a hold-fast, while the fingered hands are put to 
other uses. In the squirrels the bushy tail serves as a pro- 
tection during the winter for those parts of the body not so 
thickly covered by hair. The tail of the horse is used to 
brush away the flies that settle on the hind parts of the body. 
In other mammals, the dog, the cat, and the rat, for example, 
the tail is of less obvious use, although the suggestion has 
been made that it may serve as a sort of rudder when the 
animal is running rapidly. In several other cases, as in the 
rabbit and in the higher apes, the tail is very short, and is of 
no apparent use; and in man it has completely disappeared. 
A peculiar case of adaptation is the so-called basket on the 
third pair of legs of the worker honey-bee. A depression 
