Croonian lecture—Aspects of animal locomotion

The power of locomotion is, perhaps, one of the most striking attributes of animal life. It occurs in all groups of animals and is characterized by two conspicuous features: (i) In no other biological activity is an animal brought into closer and more intimate contact with its environment. (ii) Closely related animals may display striking differences of locomotory pattern yet in every cast the animal is able to deal precisely and efficiently with mechanical problems of great complexity. For many years, the study of animal locomotion has been concerned with two, apparently distinct, types of problems. First, attention has been paid to the mechanical or kinematic principles which animals employ in order to progress from one place to another. In many terrestrial animals these principles are relatively simple, for their limbs represent levers of one type or another; in other cases the mechanical principles are more obscure—we know little concerning the kinematics of movement of a fish or a snail, and little or nothing of the forces which propel a bird actively through the air. These problems have long attracted attention and it is encouraging to know that they are now being attacked by methods as precise and as controlled as those employed by aeronautical or marine engineers. The second type of problem is of a different nature; it is concerned with physiological nature of the locomotory machine. What is the nature of the neuro-muscular mechanism which enables and animal to utilize its muscular energy with such conspicuous precision and efficiency? How far are the movements dependent on the higher nervous centres, and how far are they dependent on the receipt of time signals from the outside world?