50 THE FOUNDATIONS OF ZOOLOGY 



Since we accept the utility of steam-engines as a fact that 

 does not call for explanation, we say we understand them when 

 we have discovered that they do neither less nor more than their 

 mechanical structure would lead us to expect. It is also clear 

 that we might understand them, in this sense of the word, even 

 if they grew, like animals, ready made ; although it is equally 

 clear that we should ask, in this case, how they became fitted for 

 human needs; and that we should not admit that we understand 

 them so long as this question is unanswered. So it is, not only 

 with the works of man and other living things, but with the liv- 

 ing things themselves. All they do may sometime prove no more 

 than might be expected from their physical basis ; but this proof 

 would not show why the things they do are useful to the beings 

 that do them, or to their species. 



While there is nothing novel in Herbert Spencer's well-known 

 dictum, that life is adjustment, it should help the modern reader 

 to grasp the significance of Aristotle's teaching, to the effect that the 

 essence of a living being is not protoplasm, but purpose. A living 

 being is a being with properties which are useful to the possessor 

 or to his species. 



If, like Paley, I kick a stone, I may change its position, raise 

 its temperature, and bring about other changes that might all be 

 computed from a few simple data. What happens if, instead of 

 a stone, I kick a dog? 



In addition to certain changes which are obviously mechanical, 

 like those in the stone, I start a new set of changes which could 

 never be computed from the study of the kick alone. But note 

 this remarkable fact : Show me the dog, and I may be able to 

 tell you what he will do. If he have short hair, a pink skin, a 

 big occipital crest, great cheek muscles, a long mandibular bone, 

 a short nose with little pigment, small red eyes and crooked legs, 

 he will not act like a dog with silky ears, curly hair, large dark 

 eyes, a long, black pointed nose, a bushy tail, and long legs with 

 big feet. 



What has the color of a dog's nose or the size of his feet to 

 do with the effect of the kick ? Obviously, nothing at all ; but 

 the changes in the dog which follow the kick are not its effect, 

 for they might follow an unsuccessful attempt to kick precisely as 



