viii PREFACE 



list. Thanks for permission to reproduce figures are especi- 

 ally due to Prof. T. H. Morgan and Prof. E. B. Wilson of 

 Columbia University, to the Editors of The Quarterly 

 Journal of Microscopical Science, The Journal of Experi- 

 mental Zoology, The Journal of Morphology, and the Bio- 

 logical Bulletin, and, for the loan of original drawings for 

 reproduction, to Dr W. E. Agar and Dr C. B. Bridges. 



L. D. 



CAMBRIDGE, 



August, 1919. 



