Theories of Evolution 21 



some peculiarity being found in one of the 

 parents and lacking in the other. The second 

 are two-sided, as all the characters are pres- 

 ent in both parents but are found in a different 

 condition. Active in one of them, it is con- 

 cealed or inactive in the other. Hence a pair 

 of contrasting units result, while in unisexual 

 crosses no pairing of the peculiar character un- 

 der consideration is possible. This leads to the 

 principal difference between species and varie- 

 ties, and to an experimental method of deciding 

 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- 



