X PREFACE 



In the treatment of genetics the simple phenomena have been described 

 in less space than formerly but, it is believed, with greater clarity. 

 Description of the mechanism in advance of its operation, a method 

 used with success in genetics courses, should contribute to this result. 

 The two linkages have been restored to the general text — at the request 

 of teachers and in conformity with the authors' preference. If the work 

 in genetics is to be shortened in any institution, this can still be done by 

 omitting the later parts of the chapter, for the topics are treated in the 

 order of their importance and desirability for beginning students. The 

 problems in genetics have been modified to call for precise (usually 

 numerical) answers, not for charts or discussions. There is no reduction 

 in the thought or organization required of the student in solving them; 

 he merely gives one specific part of his conclusion instead of all of it, 

 which should facilitate checking his accuracy. 



Among the more general of the other changes should be mentioned the 

 addition of marine habitats to the chapter on ecology, a considerable 

 extension of the historical treatment in zoogeography, and an enlarge- 

 ment of the account of prehistoric man. 



To compensate in part for the increase of space that many of the 

 foregoing revisions entail, omissions and condensation have been effected 

 elsewhere. The authors will be interested to learn whether the omissions 

 are missed. 



One incidental consequence of these extensive revisions is the removal 

 of some distinct contrasts of literary style, which are seemingly unavoid- 

 able results of joint authorship. While the present style may not be 

 better, uniformity of style is surely to be desired. • 



As usual, the authors' colleagues who use this book in an elementary 

 course have been generous with suggestions for improvement. Among 

 teachers in other institutions who have furnished ideas, special mention 

 should be made of Prof. Roy D. Shenefelt, whose recommendations could 

 have come only from a well-considered philosophy of teaching. 



A. Franklin Shull. 



Ann Arbor, Mich. 

 May, 1946. 



