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 
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