Theories of Evolution 21 



some peculiarity being found in one of the 

 parents and lacking in the other. The second 

 are balanced, as all the characters are present 

 in both parents, but are found in a different con- 

 dition. Active in one of them, they are con- 

 cealed or inactive in the other. Hence pairs of 

 contrasting units result, while in unbalanced 

 crosses no pairing of the particular character 

 under consideration is possible. This leads to 

 the principal difference between species and va- 

 rieties, and to an experimental method of decid- 

 ing between them in difficult and doubtful cases. 



Having thus indicated the general outlines of 

 the subjects I shall deal with, something now 

 may be said as to methods of investigation. 



There are two points in which scientific in- 

 vestigation differs from ordinary pedigree-cul- 

 ture in practice. First the isolation of the 

 individuals and the study of individual inheri- 

 tance, instead of averages. Next comes the task 

 of keeping records. Every individual must be 

 entered, its ancestry must be known as com- 

 pletely as possible, and all its relations must be 

 noted in such a form, that the most complete 

 reference is always possible. Mutations may 

 come unexpectedly, and when once arisen, their 

 parents and grand-parents should be known. 

 Records must be available which will allow of 

 a most complete knowledge of the whole ances- 



