XI i PREFACE 



with to construct a theory of heredity, though much was 

 wanting to render it complete. 



In my last essay I certainly suggested the possibihty of 

 solving one of the most difficult problems in heredity — viz., 

 the co-operation of the hereditary substance of the parents 

 in sexual reproduction — by assuming the existence of these 

 ' ids ' ; but I did not for a moment suppose that in doing 

 so I had propounded a complete afid elaborated theory of 

 heredity, as some of my readers have thought to be the 

 case ; much still remained to be done first. I had as yet 

 not touched upon such phenomena of heredity as have no 

 direct bearing on the question of sexual reproduction, and 

 had also abstained from any mention of the fundamental 

 point of my theory of heredity — namely, the constitution oj 

 the ids. Although I pointed out that they must possess 

 a complex structure which undergoes gradual and regular 

 changes during the development of the individual from the 

 egg-cell, I did not enter into any further details. This 

 question remained in abeyance, for I was by no means sure 

 whether the conception that I had formed on a priori 

 grounds of the minute structure of the ids would prove 

 tenable when viewed in the light of all the many phenomena 

 of heredity.* No conclusion could be arrived at respecting 

 the structure of the ids till these phenomena had been 

 individually considered. 



All my investigations on the problem of heredity were so 

 far only links, to be some day united into a chain which had 

 as yet no existence. The question of the ultimate elements 

 on which to base the theory was the very point on which I 

 remainetl longest in doubt. The ' pangenesis ' of Darwin, 

 as already mentioned, seemed to me to be far too inde- 

 pendent of facts, and even now I am of the opinion that the 



