viii PREFACE 



as to the value for their own work of many special 

 fields of boilogical inquiry. What is fundamental 

 in zoology and botany is not so extensive, or so in- 

 trinsically difficult, that a man equipped for his 

 profession should not be able to compass it. 



In the following pages we have attempted to sepa- 

 rate those questions that seem to us significant 

 from that which is special or merely technical. We 

 have, of course, put our own interpretation on the 

 facts, and while this may not be agreed to on all sides, 

 yet we believe that in w^hat is essential we have not 

 departed from the point of view that is held by many 

 of our co-workers at the present time. Exception 

 may perhaps be taken to the emphasis we have laid 

 on the chromosomes as the material basis of in- 

 heritance. Whether we are right here, the future — 

 probably a very near future — will decide. But it 

 should not pass unnoticed that even if the chromo- 

 some theory be denied, there is no result dealt with 

 in the following pages that may not be treated inde- 

 pendently of the chromosomes; for, we have made 

 no assumption concerning heredity that cannot also 

 be made abstractly without the chromosomes as 

 bearers of the postulated hereditary factors. Why 

 then, we are often asked, do you drag in the chro- 

 mosomes? Our answer is that since the chromo- 

 somes furnish exactly the kind of mechanism that 

 the Mendelian laws call for; and since there is an 

 ever-increasing body of information that points 

 clearly to the chromosomes as the bearers of the 



