172 
R. Ruggles Gates. 
MUTATIONS AND EVOLUTION. 
By R. Ruggles Gates. 
CHAPTER VI. 
Presumptive Mutations in Wild and Cultivated Plants. 
O BVIOUSLY, if the mutation theory is to be of any use the 
conceptions derived from controlled breeding experiments 
must be applicable to plant and animal species and varieties as they 
occur in nature. A beginning has only been made in this vast field 
of application, but it is sufficient to show that the mutationist 
conception of germinal changes is widely applicable to the variations 
observed among wild species. As an aid in the analysis of the 
populations that go to make up a wild species it is undoubtedly of 
great service, and its illuminating influence is also beginning to 
make itself felt in the fields of systematics, phylogeny and relation¬ 
ships. Innumerable records of species, varieties and forms in the 
literature of systematic botany and zoology and the journals of 
naturalists show how universal is the incidence of this principle. 
That the relationships and origin of nearly related species can 
also in many cases be most clearly interpreted in terms of unit 
differences has been shown by Bateson (1913) for the North 
American warblers (Helminthophila) and flickers (Colaptes) and 
other forms. In plants, the same principle has been applied (Gates, 
1916) to species of Spiranthes, Clintonia, Streptopus, Maianthemum, 
Ranunculus, Actaea and Spiraea, and in a preliminary way to the genus 
Trillium (Gates, 1917b). Similar analytical conceptions have been 
applied to the differentiation of species and varieties, their relation¬ 
ships and phylogeny, in the North American Melanthaceae (Gates, 
1918a) and Convallariaceae (Gates, 1918b). The results are in no 
sense revolutionary, but wherever the mutational principles are 
applicable they do appear to give a more clear-cut and accurate 
analysis of specific differences, and also 'of the phylogenetic and 
distributional relationships within a group. Moreover, there is the 
beginning of a physiological element in classification on these 
principles, which is a distinct gain. More recently Small (1917-19) 
has successfully applied these principles to a study of the develop- 
♦ ment of the Composite. 
It is obvious that in older groups, and wherever extinction has 
taken place, the actual succession of mutations in the development 
of any group will be obscured or lost altogether. These principles 
can be most readily and widely applied to modern expanding groups 
