CHAPTER XIV 



GENERAL CONSIDERATIONS OF ZOOLOGICAL FACTS 

 AND THEORIES 1 



i. HEREDITY AND EVOLUTION 



a. Facts 

 (i) The Distribution of Animals in Space Zoogeography 



THE earth's surface has an area of about two hundred million 

 square miles, five eighths of which is covered by the sea. This 

 vast territory is not uniform, but presents a great number 

 of sets of conditions. The major habitats are the solid earth, 

 the liquids upon the earth, and the atmosphere. Since proto- 

 plasm is impressionable and retains impressions, organisms are 

 modified by and adjusted to these different conditions; e.g. 

 Paramecia in water of a certain kind, the earthworm in ground 

 that is not too sandy, and the honeybee in the air near flowering 

 plants. The facts of zoogeography have led to the formulation 

 of the three following laws: (i) the law of definite habitats, 

 (2) the law of dispersion, and (3) the law of barriers and highways. 



THE LAW OF DEFINITE HABITATS. Among the most impor- 

 tant physical factors that determine the habitat of an animal are 

 temperature, water, density, light, molar agents, and food. The 



1 The author realizes the difficulty of doing justice to the facts and theories 

 of evolution and heredity, the social life of animals, reflexes, instincts, and 

 the animal mind in one short chapter ; nevertheless, he has endeavored to 

 - give a concise account of these subjects, believing that the majority of 

 students who take the introductory course in biology do not continue with 

 the more advanced courses and therefore have no other opportunity of be- 

 coming acquainted with this important phase of zoology. No doubt in 

 many cases the aid of a teacher will be necessary for a clear understanding 

 of the subjects treated. 



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