vill PREFACE 
as to the value for their own work of many special 
fields of biological 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 what 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 
