VARIATION 43 



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B. Continuous geno-variation, heritable variation. 



C. Discontinuous non-heritable variation, modification. 



D. Continuous non-heritable variation, modification. 

 We see that, if modification, the effect of the non-genetic 



factors, the influence of the environment, is not transmittable, 

 we have in our search for the causes of inheritable variability 

 to limit ourselves to geno-variation. But as we saw, it is not 

 easy to distinguish geno-variation from modification. The idea 

 that discontinuous variation and hereditable variation are 

 synonymous and that continuous variation is modification, is 

 surely erroneous. If we see a certain variation in any group of 

 animals or plants, it is not at all easy to find out how much of 

 the difference between the individuals is due to a difference of 

 genotype, and how much to the effect of the conditions the 

 developing individuals have encountered. 



The question as to the causes of variation can be formulated 

 as follows: What are the causes for the production of new 

 combinations of genes? And to answer this question, it is ne- 

 cessary to review what we at the present moment know about 

 genetic developmental factors. As yet we have always spoken 

 of heritable developmental factors. We now want to use the 

 ferm gene, for these factors as proposed by Johannsen. 



It must here be stated clearly, that, whereas all heritable 

 factors in the development of an organism are genes, we know 

 about several genes which are in many instances not factors in 

 the development of the individual which carry them. 



We saw that some genes happen to influence the develop- 

 ment of organisms in a striking way, for instance those genes 

 which are links in a chain of factors which leads to pigment- 

 formation. In individuals in which such a gene is lacking, there 

 is no production of pigment, and therefore such individuals are 

 strikingly different from others which have the same gene. 

 But we have seen, that this difference between two individuals, 

 with and without the same gene is not always so marked. We 

 saw that some genes have so little influence in certain combin- 

 ations, that it is hardly possible to distinguish individuals 



