54 EVOLUTION OF LIVING ORGANISMS. 



great general importance, since hybridisation, or the 

 crossing of different " species," can play but a small part 

 in evolution, and need hardly be considered as a factor 

 at all. 



The term " unit " character is apt to be misleading. 

 For practical purposes it is convenient to fix the attention 

 on some conspicuous result, such as the colour of a flower 

 or the height of a plant, but it must always be remem- 

 bered that the influence of the factor may possibly 

 always does extend over the whole organism. 



In Chapter III. it was shown that two kinds of varia- 

 tion can be distinguished : modifications and mutations. 

 The latter alone are due to changes in the inheritance, 

 and may be further classified into retrogressive mutations, 

 due to the loss of some factor, and progressive mutations, 

 due to the gain of some factor.* 



Of the origin of progressive mutations we know practi- 

 cally nothing. So far as experiment has shown, the multi- 

 tudes of domestic varieties of our plants and animals are 

 all, or almost all, of a retrogressive kind ; that is to say, 

 are due to the separation and rearrangement of the factors 

 of inheritance already present, not to the addition of new 

 factors. New characters may thus come out, but the 

 unit factors remain the same though apparently reduced 

 in number. 



Similar retrogressive mutations are known to occur in 

 nature. Common instances are albino animals and white- 

 flowered plants. The botanist De Vries, who has made 

 a special study of these variations, has collected and 

 studied a large number of cases. But it cannot be that 

 all mutations are of this negative nature. All the differ- 

 ences that distinguish man from the primitive stock 

 whence arose the whole animal kingdom can hardly be 



* Mutations, whether progressive or retrogressive, are not necessarily 

 related, as might be supposed, to the complicated changes undergone by 

 the chromatin in the maturation of the germ-cells, since they occur in the 

 Bacteria, and among other forms which reproduce asexually. 



