INTRODUCTORY 3 



to what the future may disclose whether by way of limitation 

 or extension of Mendelian method, it can be declared with 

 confidence and certainty that we have now the means of be- 

 ginning an analysis of living organisms, and distinguishing many 

 of the units or factors which essentially determine and cause the 

 development of their several attributes. 



Briefly put, the essence of Mendelism lies in the discovery 

 of the existence of unit characters or factors. For an account 

 of the Mendelian method, how it is applied and what it has 

 already accomplished, reference must be made to other works. 1 

 With this part of the subject I shall assume a sufficient ac- 

 quaintance. In these lectures I have rather set myself the task 

 of considering how certain problems appear when viewed from 

 the standpoint to which the application of these methods has 

 led us. It is indeed somewhat premature to discuss such ques- 

 tions. The work of Mendelian analysis is progressing with 

 great rapidity and anything I can say may very soon be super- 

 seded as out of date. Nevertheless a discussion of this kind 

 may be of at least temporary service in directing inquiry to the 

 points of special interest. 



THE PROBLEM OF SPECIES AND VARIETY 



Nowhere does our new knowledge of heredity and variation 

 apply more directly than to the problem what is a species and 

 what is a variety? I cannot assert that we are already in a 

 position to answer this important question, but as will presently 

 appear, our mode of attack and the answers we expect to re- 

 ceive are not those that were contemplated by our predecessors. 

 If we glance at the history of the scientific conception of Species 

 we find many signs that it was not till comparatively recent 

 times that the definiteness of species became a strict canon of 

 the scientific faith and that attempts were made to give precise 

 limits to that conception. When the diversity of living things 

 began to be accurately studied in the sixteenth and seventeenth 



1 In Mendel's Principles of Heredity (Cambridge University Press, 1909) 

 I have dealt with this subject, giving an account of the principal facts discovered 

 up to the beginning of 1909. 



