304 THE RIDDLE OF THE UNIVERSE. 



Critical comparison of sense-action in man and the 

 other vertebrates has brought to light a number of 

 extremely important facts, the knowledge of which we 

 owe to the penetrating research of the nineteenth 

 century, especially of the second half of the century. 

 This is particularly true of the two most elaborate 

 "aesthetic" organs, the eye and the ear. They 

 present a different and more complicated structure in 

 the vertebrates than in the other animals, and have 

 also a characteristic development in the embryo. 

 This typical ontogenesis and structure of the sensilla 

 of all the vertebrates is only explained by heredity 

 from a common ancestor. Within the vertebrate 

 group, however, we find a great variety of structure in 

 points of detail, and this is due to adaptation to their 

 manner of life on the part of the various species, to 

 the increasing or diminishing use of various parts. 



In respect of the structure of his sense-organs man 

 is by no means the most perfect and most highly- 

 developed vertebrate. The eye of the eagle is much 

 keener, and can distinguish small objects at a distance 

 much more clearly than the human eye. The hearing 

 of many mammals, especially of the carnivora, 

 ungulata, and rodentia of the desert, is much more 

 sensitive than that of man, and perceives slight noises 

 at a much greater distance ; that may be seen at a 

 glance by their large and very sensitive cochlea. 

 Singing birds have attained a higher grade of develop- 

 ment, even in respect of musical endowment, than the 

 majority of men. The sense of smell is much more 

 developed in most of the mammals, especially in the 

 carnivora and the ungulata, than in man ; if the dog 

 could compare his own fine scent with that of man, he 

 would look down on us with compassion. Even with 



