THE MEANING OF EVOLUTION 271 



limb-girdles, and limb-bones of a vertebrate, is a rigid 

 and fixed series of supports to which the muscles are 

 attached. Organs of locomotion are, for instance, the 

 appendages of a crustacean, the wings of a bird or 

 insect, the tail and fins of a fish, or the limbs of a 

 vertebrate. Organs of aggression are the mandibles 

 of a spider or blood-sucking fly, the chelate claws of a 

 crab or lobster, the jaws of a fish, or the claws and 

 teeth of a terrestrial vertebrate. Organs of prehension 

 and mastication are in the main also those of aggression. 

 All these parts consist of modified skeletal structures, 

 teeth, claws, etc., attached to muscles which originate 

 in the rigid parts of the skeleton. When we speak 

 of the movements of an animal we speak of the 

 motions of such parts as we have mentioned ; other 

 parts do indeed move — the heart pulsates, the 

 lungs dilate and contract, and the blood and other 

 fluids circulate through closed vessels ; but these are 

 movements of the parts of the animal, and are 

 comparable rather with those movements of the 

 plant organism that we have considered. They are 

 not to be regarded as examples of the mobility of 

 the animal in the sense of the exercise of its 

 sensori-motor system. 



A central and peripheral nervous system is, of course, 

 bound up with a motor system. Receptor organs, 

 eyes, olfactory, auditory, tactile organs of sense, and 

 so on, are the means whereby the animal is affected by 

 changes in its environment — it need not be cognisant 

 of, or become aware of, or perceive these impressions 

 on its receptor organs. These stimuli are transmitted 

 along the sensory, or afferent, nerves to the central 

 nervous system : this is the way in. The effector 

 nervous organs are the motor plates, that is, the nervous 

 structures in the muscles in which the nerves terminate. 



