1. Till-: CO^XEPTIOX OF SPECIES ACCORDlX(; 

 TO THE THEORY OF MUTATlOX. 



§ I. SYSTEMATIC BIOLOGY AND THE THEORY OF 



MUTATION. 



In every case in wliicli we were al)le to obtain a deeper 

 insight into the nature of the hereditary character of an 

 organism by direct observation or experiment we ha\e 

 found this character to be of a compound nature. Xo 

 plant transmits its pecuharities to its offspring as an in- 

 separable whole, as has been the general view until n()w. 

 On the contrary we have described a long series of phe- 

 nomena in which a single character or a smaller or larger 

 group of them can be separated from the rest and behave 

 in an entirely different way. When new species or vari- 

 eties originate, it Is not the whole nature of the organism 

 that is changed; on the contrary everything remains in 

 a state of rest except at one or two points, and it is only 

 to the changes of these points that all the improvement 

 is due. In hybridization the two types which se.xually 

 unite are always alike in the vast majority of their char- 

 acters, and the differences between them are limited to 

 a few definite units, which, in the simplest cases, can be 

 dealt with numerically. 



The analysis of organisms, therefore, leads us to the 

 hypothesis of units, which are in many respects analogous 

 to the molecules of the chemist. Thev are, however, of 



