VERTEBRATES 273 



vessels, stomachs, and intestines the same as you? They 

 all do except the fishes, which have gills in place of lungs. 



As to reproduction, you learned in your study of plants 

 that the sex method is used in some of the very lowest 

 plants and in all of the high ones. But all plants also use 

 other methods. In the higher animals, however, there is 

 only the sex method. Sperms are produced by the males, 

 and eggs by the females, and these must be united in the 

 mating of the sexes. The fertilized egg grows directly 

 into a new individual, and there is no resting-stage that 

 corresponds with the resting-stage you have observed in 

 seeds. 



The great bulk of the body of all these animals, whether 

 it be a fish or a frog, a bird or a mammal, is devoted to the 

 work of securing food and making use of the food after it 

 is obtained. In that respect they are just the same as 

 plant-bodies, although they are so differently organized into 

 definite organs. 



The intelligence of animals is related to the size of their 

 brains. All the vertebrates have bony skulls which pro- 

 tect the brains within them. As we progress from fishes 

 toward man, we find the brain growing larger and larger, 

 until in man we find developed that wonderful organ that 

 makes all the difference between man and beast. It is 

 only in brain-power that we excel the lower animals. They 

 can excel us in all the other organs. Their hearts, muscles, 

 stomachs, lungs, and sense-organs may all be better than 

 our own. It is only in our more highly developed nervous 

 tissues that we excel them. We can think, reason, and 

 imagine. And this makes us responsible for our own acts. 

 The lower animals are safely guided by their instincts. 

 We are not. All our life long we must seek self-control. 



