INTRODUCTION xi 



The whole problem centres around the interpretation of terms, and the value 

 attached to each. But whatever value be attached, the underlying causes remain. 

 The multiplication of forms may in some cases, as in Willowherbs, Willows, See., 

 be due to hybridization, in others to the normal causes of variation. 



When a species is not adapted to cross-pollination, but is self-pollinated, and 

 produces a larger range of forms, they are not the result of natural selection, but 

 rather, as De Vries explains in regard to such mutations, or species in the making, 

 due to the production of mutants in the early history of the plant, differing in one 

 or more characters from the type, and perpetuated by the addiction of the plant to 

 self-pollination. By degrees the creation of small variations from the original will 

 bring about great divergence between the parent stock and the latest mutants, until 

 the extreme forms may rightly be considered distinct species, especially if the 

 connecting forms have failed to survive. Much then depends, in our conception of 

 species, upon the survival of intermediates, or the reverse. The existence of natural 

 selection, indeed, largely hinges upon the occurrence of such a phenomenon as 

 survival, or the reverse, or in other words of adaptations to meet special conditions. 



Different from continuous variations are those produced by a change in the 

 environment. Such small continuous variations were emphasized by Darwin as 

 being advantageous and in the struggle for existence liable to be selected, other 

 plants which do not possess them not surviving. The extremes survive, hence the 

 difficulty experienced in tracing the evolution of species owing to the disappearance 

 of connecting forms. 



So far as the results of artificial selection go, in the cultivation of species, 

 Johannsen has not found that divergent forms arise; the usual result is the pro- 

 duction of a pure line with a higher average weight of seed (the special character 

 experimented with), and beyond this no great variation occurs. 



But there are other variations of kind rather than degree, and one obtains sports 

 (as in the case of white flowers), discontinuous variations, or mutations. According 

 to Darwin such sports or mutations are rare. His experience is in direct opposition 

 to that of De Vries, who worked upon QSnothera, the parent plant of his mutants 

 being, however, of doubtful nature. Whether mutations are frequent or not, species 

 may arise suddenly. Owing to the influence of cross-pollination, indeed, it is natural 

 that mutants should, once originated, be quickly altered, and their origin obscured, 

 by the influence of crossing, which is, as a rule, the prevalent type of pollination. 

 Mutations would also become crossed with typical forms, and rare mutants would 

 be eliminated. 



Self-pollination would, on the contrary, help to perpetuate mutations, and the 

 rarer, more extreme form of pollination, or cleistogamy, whereby outside influence 

 of insect visitors is entirely excluded, would have the effect of establishing any 

 mutations that could arise and in due course of producing plants entitled to rank 

 as sub-species, or even new species. 



There is in general a tendency in plants to preserve specific identity, given 

 uniform conditions of growth. The plant itself has a tendency to avoid variation 

 as a rule, a fact reflected in the prepotency of pollen or the readiness with which 

 one individual of a species is crossed with another of the same species, rather than 

 with that of another species, or even genus. Also like individuals of the same 

 species are more likely to be crossed than dissimilar individuals. This tendency is, 

 however, discounted by the erratic conduct of the agents that effect cross-pollination, 

 namely insects, though it has been shown by Lord Avebury that, as a rule, insects 

 have a marked preference for visiting plants of the same species in their wanderings 

 to and fro in search of pollen or honey. But, apart from colour and form of flower, 



