VARIATION. 5 \ 



Through the work of Mendel's followers we now know that a 

 redistribution of genes is ordinarily only possible at the form- 

 ation of gametes. We must imagine all the cells of a plant or 

 animal to contain the same set of genes, with the exception of 

 the germ-cells, and we have good reason to believe that every 

 cell of an individual is pure for the same genes atid impure for 

 the same other ones. 



Therefore, if we have a clone, a group of plants or animals, 

 which have all been derived from one single zygote through a 

 sexual process, division, budding, such a group can ordinarily 

 vary only in one way, namely through the action of non-inher- 

 ited developmental factors. Is the variation which we observe 

 within a clone, of such a nature as to make it possible, that 

 within such a group new species could arise? 



If we observe the variation of such common plants as the 

 hawthorn or Taxus baccata, or the holly in hedges or in woods, 

 we see that it is very large indeed. If we should be asked off- 

 hand to give our opinion about the proportion of this total var- 

 iation which we think is due to difference in genotype and to a 

 varying influence of the environment, we might easily be led to 

 believe that the fluctuating continuous variability observed 

 was in the main due to differences in the condition under 

 which the trees had grown, differences in soil, in shade, in hu- 

 midity. 



Now, in these three plants, clones are propagated by nurs- 

 erymen. And it is easy in any important nursery to compare the 

 variability in an ordinary mixed lot of Taxus baccata seedlings 

 to that within a row of cuttings or grafts made out of one 

 single individual. I have more than once had occasion to see 

 several rows of this tree, of which each row was a different 

 clone. And if we compare the variability of such a clone with 

 that of a mixed lot, we are struck by the fact that it is so very 

 insignificant. All the trees of the clone have exactly the same 

 habitus. Small things, such as a somewhat concical form, or a 

 somewhat more yellowish hue of the branches, or a slightly dark- 

 er tinge of the leaves are faithfully reproduced in all the indi- 



