PREFACE 



TN a book of the size to which the Cambridge 

 Manuals of Science and Literature are limited, it 

 is plainly impossible to treat in detail every aspect of 

 a subject like Heredity. One of the chief difficulties, 

 therefore, which I have encountered in preparing this 

 little book has been to decide what to leave out. To 

 some it will doubtless seem that parts of the subject 

 have been treated too fully, and other important 

 branches omitted or barely mentioned, but my aim 

 has been to give the reader a sketch of the most 

 important lines in which recent advances have been 

 made. There are many excellent works dealing with 

 the older theories — and in this subject age is measured 

 by very few years, — but our knowledge has increased 

 so greatly and is still progressing so quickly that 

 books become out of date almost as soon as they are 

 published. My attempt, then, has been to deal chiefly 

 with the quite modern developments of the subject, 

 and in order that the reader who is not very familiar 

 with the matter may feel he is on fairly sure ground, 



