VERTEBRATES. 



VERTEBRATES. 



It has become quite a standing joke among naturalists that oo one 

 knows exactly what a vertebrate is. Turn to your dictionaries and the 

 school text-books, and you will find some very exact and clear definitions, 

 which, at first sight, would seem to set the question at rest ; but they do 

 not. They lack one thing ; nature will not accommodate itself t<> them. 

 They were all very well in the early days of science, but to-day tln\ 

 nature) are all wrong. When we knew of nothing but the structure of tin- 

 adult animals (and but little of that), classification was an easy matter ; but 

 when we came to learn the steps of growth, the problem became more com- 

 plicated. It was then seen that there were various relationships i if animals ; 

 that they could not be arranged in a straight line, but that one needed the 

 geometry of three dimensions to show the position of any one form with 

 regard to its fellows. Still in a popular book we do not need to go into 

 that detail which would be necessary in a manual of science, ami hence we 

 can give all the definition which is necessary for our purposes, without pay- 

 ing any attention to the numerous exceptions. 



Vertebrates, then, comprise these forms familiar to all under the names 

 of fishes, reptiles, birds, and mammals. They are characterized by a 

 jointed backbone, or in its place a simple cartilaginous rod ; they breathe 

 either by gills or by lungs, according as they live on the land or in the 

 water. Their whole central nervous s}-stem is situated above the alimen- 

 tary tract, and the heart is below, while in most forms two pairs of appen- 

 dages — the paired fins of fishes, the legs of mammals and reptiles, and 

 the legs and wings of birds — are readily discernible. 



So far as intelligence goes, many of them stand below the higher forms 

 which we have passed, while in structure they exhibit but little compJ 

 tion. Others, on the other hand, are decidedly specialized, and one m< 

 ber — man — towers far above all the rest of the animal kingdom, 

 intellectual capacity. 



