PURITY OF THE GERM-CELLS 73 
cludes that there is no reason for supposing that these 
have undergone any modification at all. 
If we consider the cells which build up an adult organ, 
and for the moment regard each separate cell as an 
individual, we see that each of these individuals 
possesses an ancestry of cells stretching right back to 
the fertilized ovum—the single cell in which the whole 
organism originated. So far as the later cell divisions 
are concerned, the cell-lineage of a particular organ is 
separate and distinct from that of the cells of any 
other organ. At a certain distance back in the history 
of the organism we shall come across a common cell- 
ancestor for the cells belonging to a pair of neighbour- 
ing organs, and the more widely separate the parts to 
which the cells we are considering belong the further 
back must we go before we find their ancestry merging 
in a single cell. In asimilar way as with other organs, 
so it is found that the sexual cells or germ-cells of an 
adult organism have a history quite distinct from that 
of the cells of any other part of the body; and these 
cells are the only ones which are concerned in the 
formation of the offspring. Thus we see that the 
particular cell-lineage leading up to the germ-cells is 
the only one which is continued into another genera- 
tion; all the others terminate with the death of the 
individual creature of which they form a part. From 
this point of view we may consider the nature of a 
given series of animals as being determined only by 
the particular series of cells which constitute the 
direct ancestry of the germ-cells in each individual ; 
the cells which make up the bodily structure are the 
