I 



PREFACE XVU 



to diseases. We certainly possess a rich material on which 

 observations concerning the transmission of diseases might 

 be based, and this I have made use of so far as seemed 

 expedient. It must, however, not be forgotten that the 

 transmission of so-called hereditary diseases is not always 

 due to a true process of heredity, but in some cases, at any 

 rate, results from an infection of the germ. Unfortunately, 

 we are not always able to distinguish between these two 

 causes ; and as long as this is the case, the data furnished 

 by diseases can only be used with great caution, as will be 

 shown in Chapter XII. 



The manuscript of this book was practically completed by 

 the end of April last, but as the translation had then to be 

 made, its publication was delayed for some months. This 

 will account for the fact that no mention, or only a brief 

 one, has been made of researches which have appeared in 

 the interval. My sincerest thanks are due to the translator 

 — Professor W. N. Parker, — whose task has been by no 

 means an easy one : apart from the mere knowledge of 

 the two languages, an intimate acquaintance with the facts 

 treated of and with the whole science of biology is essential 

 in order to render the meaning of this complicated subject 

 clear, and at the same time to reproduce the original text 

 with anything like accuracy. I am of course unable to 

 /udge how far Mr. Parker has succeeded in clothing my 

 ] deas in good English, but am glad to state that they have 

 leen given very correctly, so far as I can judge from those 

 )arts which we have discussed together. 



In conclusion, I must express my warmest thanks to the 

 jovernment under which I have the good fortune to live, 

 for the efficient way in which they have seconded my 

 endeavours, by releasing me from my academical duties 



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