INTRODUCTION. 



By the Mutation theory I mean the proposition that 

 the attributes of organisms consist of distinct, separate 

 and independent units. These units can be associated 

 in groups and we find, in alhed species, the same units 

 and groups of units. Transitions, such as we so fre- 

 quently meet with in the external form both of animals 

 and plants, are as completely al)sent between these units 

 as they are between the molecules of the chemist. 



It is perhaps unnecessary to remark that these gener- 

 alizations refer to the animal as well as to the vegetable 

 kingdom. In this book, however, I shall confine myself 

 to the latter, in the belief that the truth of the principle 

 will be granted in the case of the former as soon as it 

 has been shown to apply in that of plants. 



The adoption of this principle influences our attitude 

 towards the theory of descent by suggesting to us that 

 species have arisen from one another by a discontinuous, 

 as opposed to a continuous, process. Each new unit, 

 forming a fresh step in this process, sharply and com- 

 pletely separates the new form as an independent species 

 from that from which it sprang. The new species ap- 

 pears all at once ; it originates from the parent species 

 without any visible preparation, and without any obvious 

 series of transitional forms. 



