634 | ANNUAL REPORT SMITHSONIAN INSTITUTION, 1911. 
structed for the effort exerted at different speeds by the horse, as has 
been produced by Marey for the man, only the distribution of energy 
would probably be very different. 
Turning next to other animals, it is interesting to observe that a 
greyhound gets its high speed in proportion to its size owing to the 
great flexibility of its long body, which enables it to draw its hind 
legs forward each time for the next bound, and also bound forward 
both from its fore and hind legs. The other animals in galloping 
have each the same general kind of movement, although the deer, 
curiously enough, only bounds from its hind legs, and differs in this 
respect from the horse; and also it will be noticed the want of 
flexibility in the body of an animal may be one of the causes of its 
relatively slow speed. But whether it be man, horse, dog, or any 
other animal, the same characteristic is found, namely, that locomo- 
tion, apart from the bounding action, takes place by a sort of rolling 
action on the ground. The idea which had persisted since the 
delineation of horses in Assyrian and Egyptian pictures, that both 
the fore or both the hind legs are put on the ground simultaneously, 
is thus exploded. As Mr. Muybridge truly said: 
When, during a gallop, the fore and hind legs are severally and consecutively thrust 
forward and backward to their fullest extent, their comparative inaction may create 
in the mind of the careless observer an impression of indistinct outHnes; these suc- 
cessive appearances were probably combined by the earliest sculptors and painters, 
and with grotesque exaggeration adopted as the solitary position to illustrate great 
speed. 
As a matter of fact, each leg in turn, as it rests on the ground, 
stops for a moment just as much as in the forward position above 
mentioned, and if you watch a dog galloping you can see quite 
clearly the rolling stroke action I have mentioned. 
With the above facts in mind, we can understand exactly the 
limitations to animal locomotion. In the words of Mr. Muybridge: 
When the body of an animal is being carried forward with uniform motion, the 
limbs in their relation to it have alternately a progressive and a retrogressive action, 
their various portions accelerating in comparative speed and repose as they extend 
downward to the feet, which are subjected to successive changes from a condition 
of absolute rest to a varying increased velocity in comparison with that of the body. 
Hence, all animal locomotion absolutely lacks that continuity of 
movement, the production of which we shall see is the distinguishing 
feature andl the direct cause of the high speeds Sach aL in mechanical 
locomotion. 
The exchange of the intermittent movement of nature for one 
having the desired continuity of movement has been effected by means 
of what is possibly the greatest and yet the simplest of all human 
inventions, namely,:the wheel. 'The wheel was made and used prob- 
ably thousands of years before man learned to replace muscular effort 
