Like the bodies of other vertebrates, the human body has a brain-box 

 at the front end of the backbone. Comparing our arms and legs with the 

 hmbs of four-footed animals shows a remarkable correspondence in detail, 

 bone for bone^ (see illustration, p. 48). The resemblances extend to the 

 bones of a bird's wing or the flipper of a whale (see illustration, p. 49). 

 Muscles, blood vessels, brains and nerves, kidneys, reproductive organs, 

 sense organs, and digestive organs of all vertebrates have much in common; 

 and the human systems of organs fit the same general pattern. 



Man and Other Mammals' The five classes of vertebrates are repre- 

 sented by a perch, a frog, a turtle, a turkey, and a squirrel. When we say 

 that "man is a mammal", we mean that man has all the qualities which 

 mammals have i7i common. That is not the same as saying that man has 

 the qualities of all the mammals, which is, of course, not true. Man has 

 qualities that no other mammal has; every mammal has qualities that no 

 human being has. Man cannot climb trees like monkeys or squirrels, nor 

 live on grass like sheep and cows, nor cut through trees with his teeth like 

 the beaver. But man is able to do what all these and other mammals also 

 do — in common. He sees with the same kind of eyes, pumps blood with 

 the same kind of heart, breathes with the same kind of lungs. 



All the mammals are alike in having milk-glands, which furnish food 

 to the suckling infant. They are all "warm-blooded". The newborn indi- 

 vidual has the same general form as the adult. The skin is more or less 

 covered with hairs, at least during part of life. In all these ways man is also 

 a mammal, although he differs from all the other mammals. 



Various mammals can get up on their hind legs for longer or shorter 

 periods. But none of them regularly walk erect, as human beings normally 

 do. It has been suggested that by walking altogether on their hind legs, 

 the ancestors of the human race freed their arms and hands for other 

 activities, and were therefore enabled to develop these organs to higher 

 skills. It is true, at any rate, that, if we judge from fossil remains, ancient 

 man was an erect animal, whereas the front legs are used in moving about 

 on the ground by all the other modern primates (the "first" order of mam- 

 mals, which includes the apes and monkeys as well as man). 



How Does Man Differ from Other Primates? 



Hands and Feet The differences between man's front limbs and hind 

 limbs are related to the erect walk. The front and hind limbs are distinct 



^We must not be disturbed by so much attention to dry bones, nor attach to the bones 

 any strange virtues. Scientists use bones in many of their comparative studies only because 

 these structures can be more easily preserved and more accurately measured and compared 

 than other parts. 



-See No. 2, p. 60. 



46 



