Epilogue: What Gomes of Studying 

 Vertebrates 



Animals must eat, breathe, and move. The nature of the organs 

 which perform these functions is most directly and necessarily related 

 to the animal's environment. This is especially true of the moving and 

 breathing. In relation to these two functions, the animal's environment 

 may be, respectively, water-water, land-air, water-air, or air-air. Gill- 

 breathing fishes possess a simple locomotor mechanism adapted to 

 propelling a finely streamlined body through water. Lung-breathing 

 reptiles and mammals possess a more or less elaborately specialized 

 tetrapod mechanism adapted to locomotion over a solid substratum. 

 Flight imposes excessive demands on the bird's breathing mecha- 

 nism and requires a peculiar and very highly specialized locomotor 

 apparatus. 



The relations of vertebrates to environmental water, land, and air 

 are almost as diverse as could be imagined. Yet all vertebrates, whether 

 minnow, python, horse, or hummingbird, possess the same basic 

 structure. But it is so modified and elaborated as to fit the most diverse 

 requirements. Inspection of the whole assemblage of vertebrates 

 reveals the fact that the modifications and elaborations of a particular 

 organ or system are not desultory or unrelated to those of other organs 

 in their occurrence. No fish possesses hair or feathers; no mammal has 

 a three-chambered heart. On the contrary, it is possible to recognize 

 groups of animals which may be so arranged in series that the struc- 

 tural differences of a particular organ are continuously progressive 

 throughout the series. If this be done for each of all systems of organs, 

 it will be found that, at least in general, the groups fall into the same 

 sequence in all of the several series. Following are some examples of 

 this sequence. 



The skin of fishes secretes a thin superficial cuticula similar to that 

 of invertebrate skin. Calcareous scales, usually bony, and mucous 

 glands, usually of the simple unicellular type, are the characteristic 

 products of their skin. In amphibians cuticula is present only in the 



