PREFACE ix 



accessory chromosome, important as it may be, to 

 a time when the former concept shall have taken 

 firm root. 



A chapter on Animal Behavior was projected but 

 was abandoned when it was found that its inclusion 

 would have increased the size of the volume unduly. 

 For the same reason no apology need be offered for 

 the constant reference by name without comment 

 to the various groups of animals and plants. The 

 first-hand knowledge of the types in the laboratory 

 will have supplied the descriptive details for which 

 there is no room in the present work, although text- 

 figures have been freely used to illustrate the forms 

 mentioned. 



In such a book as the present one, little can be 

 claimed for originality except the manner of pre- 

 senting the subject. I have sought counsel and 

 criticism in those fields in which my personal knowl- 

 edge is least dependable, and I hope that such errors 

 as may have crept in* will not be significant ones. 

 I am particularly indebted to Professor George T. 

 Moore, Director of the Missouri Botanical Gardens, 

 who read the whole book in manuscript, and to Pro- 

 fessor Walter E. Garrey, who read the proof of the 

 first four chapters. Acknowledgments are also due 

 to the following for the use of cliches or permission 

 to copy figures: to Herr Gust a v Fischer, Jena, for 

 permission to use figures 7, 13, 34, 55, 60, and 82 ; 

 to Messrs. Henry Holt and Co., for the use of fig- 

 ures 8, 22, 49, 92, 94, 100, 106, and 112; to Messrs. 

 Ginn and Co., for the use of figures 31 and 103; to 



