THE METHODS AND SCOPE OF GENETICS 3 



which a given chemical system yields. Differences in development may, 

 also, occur because of actual diversities in the germinal substance, and 

 these may arise from the intermingling of different kinds of germinal 

 substance, such as obviously takes place in sexual reproduction, a 

 cause of variation which has been ably advocated by Weismann and 

 styled by him amphimixis; or they may arise from actual changes 

 in the germinal substance, distinct from the intermingling of germinal 

 elements which already exist, a form of variation which has been 

 proposed and elaborated by de Vries under the name of the mutation 

 theory. Accordingly the term variation in genetics is so defined that it 

 includes differences in individuals related by descent, although many 

 authors do not include within the term those differences which are due to 

 environmental conditions of all categories. The following definition is 

 framed in conformity to that already given for genetics. 



Variation is difference, whether in the expression of somatic characters 

 or in the elements of germinal substance, among organisms related by 

 descent. 



Heredity is commonly defined as the tendency of offspring to develop 

 characters like unto those of their parents; according to Castle it is 

 resemblance based upon descent. Thomson presents a very able dis- 

 cussion of the concept, heredity, together with criticism of definitions 

 which have been offered from time to time for the term. According to 

 his definition, by heredity is meant nothing more nor less than organic 

 or genetic relation between successive generations. 



The universal tendency of organisms to produce similar organisms 

 is the cause of the maintenance of organic groups and group relations. 

 But experimental research has demonstrated that sometimes new com- 

 binations of germinal substance produce characters which have not been 

 exhibited by parents. It is necessary, therefore, to define heredity in 

 such general terms that it will include those exceptional characters which 

 have never been exhibited by any ancestor. Now regardless of any 

 external difference which may be exhibited by an individual, its germinal 

 constitution bears a perfectly definite relation to those of its parents. 

 For that reason the following definition is stated in terms of elements of 

 the germinal substance, rather than in terms of somatic characters. 



Heredity is germinal resemblance among organisms related by 

 descent. 



Finally, with respect to the content of genetics, emphasis should 

 be laid upon the importance of a consideration of the various phases of 

 development. In development are included all those changes and cycles 

 through which the individual passes in attaining the adult condition. 

 Obviously there is much in development which cannot be treated at all 

 in an elementary text-book of genetics, for particular cycles or phases of 



