"•% 



Chap. XXVII. OF PANGENESIS. 395 



are transmitted with their newly acquired peculiarities to the 

 offspring. On any ordinary view it is unintelligible how changed 

 conditions, whether acting on the embryo, the young or adult 

 animal, can cause inherited modifications. It is equally or even 

 more unintelligible on any ordinary view, how the effects of the 

 long-continued use or disuse of any part, or of changed habits 

 of body or mind, can be inherited. A more perplexing problem 

 can hardly be proposed ; but on our view we have only to sup- 

 pose that certain cells become at last not only functionally but 

 structurally modified ; and that these throw off similarly modified 

 gemmules. This may occur at any period of development, and 

 the modification will be inherited at a corresponding period ; for 

 the modified gemmules will unite in all ordinary cases with the 

 proper preceding cells, and they will consequently be developed 

 at the same period at which the modification first arose. With 

 respect to mental habits or instincts, we are so profoundly igno- 

 rant on the relation between the brain and the power of thought 

 that we do not know whether an inveterate habit or trick induces 

 any change in the nervous system ; but when any habit or other 

 mental attribute, or insanity, is inherited, we must believe that 

 some actual modification is transmitted ; 52 and this implies, ac- 

 cording to our hypothesis, that gemmules derived from modified 

 nerve-cells are transmitted to the offspring. 



It is generally, perhaps always, necessary that an organism 

 should be exposed during several generations to changed con- 

 ditions or habits, in order that any modification in the struc- 

 ture of the offspring should ensue. This may be partly due 

 to the changes not being at first marked enough to catch the 

 attention, but this explanation is insufficient ; and I can account 

 for the fact, only by the assumption, which we shall see under 

 the head of reversion is strongly supported, that gemmules 

 derived from each cell before it had undergone the least modifi- 

 cation are transmitted in large numbers to successive genera- 

 tions, but that the gemmules derived from the same cells after 

 modification, naturally go on increasing under the same favour- 

 ing conditions, until at last they become sufficiently numerous 

 to overpower and supplant the old gemmules. 



52 See some remarks to this effect by Sir H. Holland in his ' Medical Notes,' 

 1839, p. 32. 



