PREFACE 



This book is an attempt to present, in a form as simple and 

 readily intelligible as possible, the subject of heredity, as 

 related to man and his creatures, the domestic animals and 

 cultivated plants. To write such a book has been with the 

 author a long cherished ambition, but one which, as the years 

 went by, seemed less and less likely of realization, as knowl- 

 edge of the subject increased and took on more and more 

 complicated forms. Each year, however, he has been 

 forced by his responsibilities as a teacher, to make, for 

 students having only an elementary knowledge of biology, 

 an analysis and summary of our knowledge of this subject 

 to date. The longer he has continued to do this, the more 

 fully he has realized that a subject in a state of healthy 

 growth can never assume a final and finished form. He 

 makes no apology, therefore, for presenting the subject 

 with very unevenly and incompletely developed parts. 

 Such, it must be confessed, is the present state of our knowl- 

 edge. 



It would be a great service to the student to show him 

 where in his subject positive knowledge stops and specula- 

 tion, the useful servant but dangerous master in science, 

 begins. This task, where possible, has been attempted in 

 this book. But such attempts can of necessity succeed only 

 partially and for the time being, for it often happens that 

 the speculation of today becomes the verified theory of 

 tomorrow. For having guessed right and proved the cor- 

 rectness of their guesses, we honor in this field the names of 

 Lamarck, Darwin, Weismann, and Mendel. Others still 

 living have made contributions of scarcely less importance 

 but to name them would be invidious. Americans may take 

 encouragement from the thought that all are not likely to 

 be named from one side of the Atlantic and later enumera- 



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V 



19653 



