Chap. XXVIl. OF PANGENESIS. 389 



posed ; but on our view we have only to suppose that certain 

 cells become at last structurally modified ; and that these 

 throw off similarly modified gemmiiles. This may occur 

 at any period of development, and the modification will be 

 inherited at a corresponding period ; for the modified gem- 

 mules will unite in all ordinary cases with the proper pre- 

 ceding cells, and 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 

 ignorant of the relation between the brain and the power of 

 thought that we do not know positively whether a fixed habit 

 induces any change in the nervous sj^stem, though this seems 

 highly probable ; but when such habit or other mental at- 

 tribute, or insanity, is inherited, we must believe that some 

 actual modification is transmitted ;^^ and this implies, accord- 

 ing to our hj^pothesis, that gemmules derived from modified 

 nerve-cells are transmitted to the offspring. 



It is generally necessary that an organism should be ex- 

 posed during several generations to changed conditions or 

 habits, in order that any modification thus acquired should 

 appear in the offspring. This may be partly due to the 

 changes not being at first marked enough to catch 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 unmodified unit or part are transmitted in large 

 numbers to successive generations, and that the gemmules 

 derived from the same unit after it has been modified go on 

 multiplying under the same favourable conditions which first 

 caused the modification, until at last they become sufficiently 

 numerous to overpower and supplant the old gemmules. 



A difficulty may be here noticed ; we have seen that there 

 is an important difference in the frequency, though not in 

 the nature, of the variations in plants propagated by sexual 

 and asexual generation. As far as variability depends on 

 the imperfect action of the reproductive organs under changed 

 conditions, we can at once see why plants propagated asexually 



^^ See some remarks to this effect by Sir H. Holland in his ' MedicaJ 

 Kotes.' 1839, p. 32. 



