^34 ORIGINATIVE FACTORS IN EVOLUTION: 



Among ioborn variations it is useful to distinguish between 

 mutations (Galton's " transilient variations", Bateson's "discontinu- 

 ous variations") and small fluctuating variations. The former arise 

 brusquely, with a measure of perfectness from the first, without 

 intergrades, and are markedly transmissible. The latter are of the 

 nature of " a little more or a little less ", they show intergrades ; 

 their transmissibility has not been much studied, but it has been 

 proved in a few cases. 



It IS also useful to distinguish quantitative variations from defi- 

 nite novelties. The reduction or exaggeration of a quality, the 

 dropping-out of a character altogether, a re-arranged pattern of 

 hereditary items, may be called quantitative, and may be explained 

 as due to permutations and combinations of the determinants or 

 factors of hereditary characters. For such shufflings of the cards 

 ample opportunities are afforded in the course of the maturation 

 of the germ-cells. 



Another possibility is afforded at the beginning of each individual 

 life, where, in the great majority of cases, two very complex sys- 

 tems of dual origin become a new unity which normally develops 

 into a harmonious organism. Some modern evolutionists attach 

 great importance to crossing as a cause of variations. 



But the greater difficulty is with the origin of the distinctively new, 

 of what may be called qualitative variations or mutations, (a) 

 It may be that deeply-saturating environmental influences act as 

 variational stimuli on the germ-cells, provoking change, (b) Definite 

 changes in the nuclear bodies or chromosomes of the germ-cell have 

 been proved to be associated with particular mutations in the full- 

 grown organism, and, in addition to the opportunities for chromo- 

 somic change afforded in the history of the germ-cells — before, dur- 

 ing, and after fertilisation — it is possible that chromosomes, which 

 are living units, may change suddenly like Bacteria, or may undergo 

 age-changes, or may exhibit periodic re-organisation like slipper- 

 animalcules, or rejuvenescence-changes like those occurring in some 

 cases of regeneration and asexual multiplication. 



The tendency of modern research is to emphasise the idea of 

 particulateness, for it looks as if the characteristics of organisms 

 were often made up of elementai-y units, without intergrades, as 

 sharply separated from one another as the chemical elements. But 

 we must not lose sight of the unity of the organism, even in the 

 one-cell phase of its being, and of the correlation of variations. A 



