312 THE THEORY OF THE GENE 



On the other hand, the addition of a single chromosome 

 or of two members of the same pair, or of two or more 

 members of different pairs to the group, or the loss of a 

 whole chromosome from the group, may be expected to 

 produce more evident effects on the individual. There is 

 some evidence that such additions or losses are less ex- 

 treme when many chromosomes are present, or when the 

 change takes place in a small chromosome. From the 

 point of view of the theory of the gene, this result is what 

 would be anticipated. For instance, the addition of one 

 chromosome means that a large number of genes are now 

 present in triplicate. The balance of the genes is changed 

 in the sense that there are now present more genes of 

 certain kinds than before, but since no new genes are 

 added the effects would be expected to be distributed 

 amongst many of the characters that might be somewhat 

 enhanced or diminished in intensity. This accords with 

 the facts as yet reported. It is interesting to note, how- 

 ever, that, as far as known, the general results are not 

 beneficial but, if anything, deleterious. This, too, is ex- 

 pected if the adjustments, both to internal and to exter- 

 nal relations, are as perfect as possible in the normal 

 individual as its long evolutionary history might lead 

 one to expect. 



Because such a change affects many parts to a slight 

 degree, it does not follow that such effects are more likely 

 to lead to the establishment of a new viable type than 

 when changes are brought about one step at a time by 

 changes in single genes. 



Furthermore, even the addition of two new chromo- 

 somes of the same kind, giving possibly a new stable type 

 of inheritance, does not improve the situation, but, as far 

 as we know, — the evidence is slight at present, — the mal- 

 adjustments are even further increased. For these 

 reasons it does not seem that a change from one chromo- 



