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PREFACE 



ANY attempt at the present time to work out a theory 

 of heredity in detail may appear to many premature, 

 and almost presumptuous : I confess there have been times 

 when it has seemed so even to myself. I could not, how- 

 ever, resist the temptation to endeavour to penetrate the 

 mystery of this most marvellous and complex chapter of 

 life as far as my own ability and the present state of our 

 knowledge permitted. 



Even though the present attempt may be very imperfect 

 and incomplete, I cannot regard it as premature. Our 

 knowledge has increased during the last twenty years to 

 such an extent, that it does not seem to be altogether a 

 hopeless task to inquire into the actual processes on which 

 the phenomena of heredity depend. It is, moreover, very 

 essential that we should possess a theory of heredity, worked 

 out in such a manner as to suggest new problems, which in 

 their turn will lead to new solutions. 



Previous hypotheses have been insufficient in this respect, 

 owing to the fact that they have not been worked out in 

 detail. They are rather to be regarded as paving the way 

 to future theories, by merely formulating explanatory prin- 

 ciples without professing to apply them to all the different 

 groups of phenomena which come under the head of 

 heredity, by which means alone their true value can be 

 tested. Even Darwin's theory of 'pangenesis' was inade- 



