The note enclosed in Dr. Chapman's letter is printed 

 below :— 



" Many names have been given to the representatives in 

 the germplasm of the various characters of the adult organism, 

 genes, biophores, etc. : it will be simplest to call them ' factors,' 

 as that is a term with which we are perhaps most familiar and 

 with which there has been more elaboration of hypotheses 

 than with almost any other. 



" The hypothesis that I wish to suggest with regard to 

 factors is not concerned with what they actually are. My own 

 belief is that a factor is represented in the germplasm by a 

 differentiation in the structure (chemical, organic or what not) 

 of every molecule of the plasm; that it is present in every 

 portion of the plasm. The hypothesis would perhaps be 

 more easily seized and discussed if we accepted the view that 

 a factor is something that could be separated from the rest 

 of the germplasm, and the factor present in any zygote could 

 be extracted and exhibited on the point of a needle. 



" Let it be admitted that in talking of a factor we are 

 talking of something that actually exists and may be discussed 

 in very similar terms, whether it be merely a variation in struc- 

 ture of the whole protoplasm, or as a distinct and separable item. 



" In defining a species, the most distinct character deter- 

 mining that two species are two species and not forms of one, 

 is, that they are not syngamic. We meet, however, with 

 abundant cases where this test gives a doubtful result. The 

 two forms may be more or less fertile together, and the question 

 whether they are two or one species has to be decided on other 

 grounds, or left undecided, except in the opinions of individuals 

 according to their idiosyncrasies. 



"It is suggested as a hypothesis that factors act towards 

 each other on the same principles, whatever they are, on which 

 species act. To be a little more definite, the factors for some 

 particular character, in two portions of germplasm of two 

 different individuals, may combine freely, or may refuse to 

 combine, or may present relations to each other that are 

 intermediate in different degrees between these two extreme 

 attitudes. 



