Physiological and Psychological Herettity. 277 



A curious instance of the power of reproduction in the cell is 

 found in the begonia phyllomaniaca. If a piece of the leaf of 

 this plant be taken, and planted in suitable soil, maintained at 

 a proper temperature, a young begonia will spring from it ; and 

 so small is the fragment that is capable of producing an entire 

 plant, that a single leaf may produce about one hundred plants. 1 

 Nor is this all, for each plant so produced in turn develops on 

 its shoots and on its leaves myriads of similar cells, inheriting the 

 same property of becoming, in their turn, like plants. Thus the 

 original cell, on leaving the mother plant, inherits not only 

 the power of self-reproduction, but multiplies, it, and distributes 

 it without any diminution of its energy to all the cells of the 

 plant it produces, and this for countless generations. 



To explain this power of reproduction and hereditary trans- 

 mission in living beings in general, Darwin offers the provisional 

 hypothesis of pangenesis, ' which implies that each of the atoms 

 or units constituting an organism reproduces itself. ' 



It is almost universally admitted, he tells us, that the cells, 

 propagated by spontaneous division, preserve the same nature 

 and are ultimately converted into different substances and bodily 

 tissues. Alongside of this mode of multiplication, I suppose 

 that the cells, prior to their conversion into formed and perfectly 

 passive material, emit minute grains or atoms which freely circulate 

 through the entire system, and when they find sufficient nutrition 

 afterwards develop into cells like those from which they came. 

 These atoms we will call gemmules. We assume that they are 

 transmitted by parents to their descendants, and that usually they 

 develop in the generation immediately following, though for several 

 generations they may be transmitted in the dormant state and 

 develop at a later period. It is supposed that gemmules are 

 emitted by each cell or unit, not only during its adult state, but 

 during all its states of development. Finally, we assume that the 

 gemmules have a mutual attraction for one another, and hence 

 their aggregation into germs and sexual elements. Thus, strictly 

 speaking^ it is not either the reproductive elements or the germs 



1 Herbert Spencer, Principles of Biology, vol. i. 65. 



