PREFACE XV 



In the same way we shall succeed in obtaining a firmer and 

 firmer grasp of the problem of heredity, which but a short 

 while ago appeared so utterly unapproachable. 



What in this particular question appears to afford additional 

 promise of success is the fact that we can in a sense approach 

 it from two sides ; — namely, by observations, firstly, on the 

 phenomena of heredity, and secondly, on the hereditary sub- 

 stance itself, with which we are now of course acquainted. 

 We can now form an estimate as to whether an explanation 

 of any particular phenomenon of heredity is of a merely hypo- 

 thetical nature, or whether it may attain to the value of an 

 established fact, inasmuch as we are in a position to judge, 

 within certain limits at all events, whether it is consistent 

 with the actual behaviour of the hereditary substance. 

 Hitherto this has not been possible, and hence all previous 

 theories, including both that of Darwin's gemmules and of 

 Herbert Spencer's units, were up to a certain point purely 

 speculative. We are now better off in this respect ; and I 

 have no doubt that further research will enable us to pene- 

 trate far more deeply still into the compHcated processes 

 connected with the idioplasm, if we are prepared to reason 

 on the results of our observations, and to utilise every 

 theoretical advance as an incentive to fresh questions 

 regarding the processes in connection with the distribution 

 of the mysterious nuclear substance. 



We are still far from having attained a complete insight 

 into the matter, but I trust nevertheless that the present 

 attempt at a theory of heredity is no mere work of the 

 imagination ; and though it still be no more than an attempt, 

 which will be followed by better ones, I venture to believe 

 that time will prove it to contain more definite points, 

 forming the centre of numerous possibilities, than many will 



