CHAPTER Tt 
VESTIGIAL PARTS. 
UNDER the term vestigial it will be convenient to 
consider those parts commonly described as rudimentary, 
abortive, atrophied, or useless. On the whole it is 
better to refer to these structures as vestigial. In 
dealing with such examples as the denticles of the 
narwhal, which never cut the gum, the teeth of orni- 
thorhynchus, or the splint-bones of horses, we have un- 
mistakable evidence that they are remnants of structures 
which were functional in the ancestors of these animals. 
In many instances it is not easy to decide whether 
a diminutive, or feebly grown part, in one animal is the 
remnant of an organ better grown in its ancestors, or 
the rudiment of an organ which has arrived at a higher 
degree of perfection in its descendants. 
This is illustrated in the hind limbs of whales. If 
we regard cetaceans as living representatives of land 
mammals which have taken to water, the hind limbs 
are remnants; on the other hand, if the descendants 
of cetaceans have acquired terrestrial habits, then, the 
hind limbs which do not project beyond the skin, but 
are deeply buried in the blubber, must be considered — 
rudimentary or incipient structures. The whale is 
merely selected as an illustration of the advantage or 4 
