126 THEORIES OF HEREDITY 



the ottsp^in^^^ for, accordin^^ to the hypothesis, they 

 would continue to give off gemmules. No such here- 

 ditary inlluence has ever been traced or even rendered 

 probable. 



When we incjuire why Darwin was led to frame such 

 a hypothesis, which, in spite of its great merit in con- 

 necting together a number of apparently isolated facts, 

 has so much to be said against it, we fmd the answer in 

 a reply to one of Huxley's letters, in which Pangenesis 

 had evidently been adversely criticized. Thus Darwin 

 says : * I do not doubt your judgment is perfectly just, 

 and I will try to [3ersuade myself not to i)ublish. The 

 whole attair is much too spectilative ; yet I think some 

 such view will have to be adopted, when I call to mind 

 such facts as the inherited effects of use and disuse, &c.' ^ 



This opinion of Darwin's is as true to-day as when it 

 was written at some uncertain date about the year 1865. 

 If the effects of use and disuse are transmitted, the 

 explanation must be sought in a hypothesis constructed 

 on the lines of Pangenesis. But if we are mistaken in 

 believing that such transmission occurs, a very different 

 hypothesis will account for the facts. 



The manner in which the transmission of such effects 

 can be explained by the hypothesis of Pangenesis is 

 shown in Diagram /. Two of the somatic cells, O on 

 the right side and V on the left, are dark coloured. 

 This is to represent some change WTought in their 

 structure by the influence of an external force, or by 

 some unusual exercise or practice of a part. Thus the 

 darkened Q might represent the change which occurs in 

 a bone-cell when a bony growth has been caused by 

 intermittent pressure long-continued; V on the left side 

 might represent the change which occurs in a nerve-cell 

 when some new habit is acquired by long practice. Such 

 altered cells would produce correspondingly altered gem- 

 mules, indicated by the same dark appearance : these 

 ex Jiypothcsi would be stored up in the germ-cells, and 

 w^ould reproduce similarly altered cells in the offspring. 



I have given a very brief account of the main features 

 * Life and Letters ^ first edition, 1887, vol. iii, p. 44. 



