INTEODUCTORY COLLECTION. 
27 
of a running animal, in which the limbs perform no office but 
that of supporting the body on the ground. It stands on the 
tips of the toes of its elongated slender feet. 
6. A Porpoise, adapted solely for swimming in the water. 
The fore limbs are converted into flattened paddles, and the 
hind limbs are entirely absent, their function being performed 
by the tail. The rudimentary pelvic bones are preserved. 
The rest of the case is occupied by details of the skull in some 
of its principal modifications. At the top are diagrams of the 
structure of bone and cartilage as shown by the microscope. 
In the wall-case on the opposite (north) side of the bay the 
outline of the osteology of mammals is continued by illustrations 
of the structure of the limbs. At the top of the case is a 
diagram showing the correspondence of the hand and the foot 
in its complete typical form, with the names applied by 
anatomists to the different bones. The series of specimens 
below show the principal deviations which actually occur from 
the typical condition, which, as will be seen, is very nearly 
preserved in the human hand. One interesting series shows 
some of the stages of modification for special purpose (specializa- 
tion) by which a typical five-fingered hand becomes converted 
into that of the single-fingered Horse, and another series ends with 
the hand of the ruminants, with only two fingers remaining, some- 
times, but erroneously, called a " cloven hoof." Similar changes 
are shown in the toes of the hinder extremity, illustrating the 
same common plan running through infinite modifications in 
detail, enabling the organ to perform such a variety of purposes, 
and to exhibit such diversity of outward appearance. The 
existence of this common plan is now generally thought to be 
due to inheritance from a common ancestor. 
In the lower part of the case are examples of all the different 
kinds of articulations or joints by which the various bones of the 
body are connected together. 
The central case of the bay contains a collection illustrat- Teeth of 
ing the principal characters of the teeth in the Mammalia. 
Its inspection should commence at the north-east corner, where 
the visitor will find himself after completing the survey of the 
osteological specimens in the wall-cases. In the first division 
specimens are placed showing the general characters of 
