PREFACE 



The changes introduced in this sixth edition are more than usually 

 varied. While none can be regarded as radical, they affect in important 

 ways nearly every part of the general plan. The book is still devoted to 

 principles; indeed, the changes appear even to emphasize its devotion to 

 fundamental concepts. 



If any one statement can be made which would characterize much of 

 the alteration now made, it is that the treatment of function has been 

 increased or clarified or thrown into relief by emphasis. Such changes 

 relate, among others, to enzymes, photosynthesis, oxidation, muscle 

 action (including cardiac), breathing movements, transfer of respiratory 

 gases, blood composition, the clotting process, kidney function, vitamins, 

 endocrines, the placenta, and reflex arcs. The authors have not hesitated 

 to give chemical formulas and reactions that beginning students need not 

 be expected to remember or reproduce, because these exact forms of 

 expression carry conviction concerning the precision of present knowledge 

 Avhich no more general statement can produce. 



Greater clarity of exposition has been sought at many places by illus- 

 trations and slight changes of language or inclusion of features not 

 heretofore expressly described. Comparisons that were formerly illus- 

 trated by figures borrowed from research contributions are now in several 

 instances portrayed by simplified diagrams placed side by side with the 

 contrasts indicated. Among the phenomena thus treated are symmetry, 

 centralization and cephalization of the nervous system, endocrine secre- 

 tions, the hydroid metagenetic cycle, and the evolution of living things 

 in geological time. More explicit description is the method adopted for 

 the types of circulatory and excretory systems, for the operations of the 

 kidney, for the biogenetic law, and others. 



Order and emphasis have occasionally been changed at the suggestion 

 of teachers elsewhere, even when the authors were not quite convinced 

 that the new method was an improvement but could see no objection to 

 it. Molecules and atoms have been introduced before protons, neutrons, 

 and electrons. The names of the phases of mitosis have been restored 

 in the belief that under the guidance of a good teacher cell division will 

 still be conceived as a continuous process. Biological terms have been 

 introduced in a number of places with the conviction that names some- 

 times clarify ideas, simply because terms must have definitions. Yet the 

 glossary is today shorter than in the early editions. 



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