INHERITANCE OF WOOL-CHARACTERS IN SHEEP                371

      2.  The females of the P₁ generation should be of the same age — prefer-
          ably not primiparous, and the age of the male should not vary by
          more than ±1 year from the age of the females.
      3.  Examination and analysis of fleece-characters in the parental generation
          should be carried out on 12 months' growth of wool, or if this is not
          possible, samples should be taken at the same time from all the sheep
          so that comparable results may be obtained and variations reduced
          to a minimum. Upon no account should samples be taken from the
          first year's growth of wool in F₂ and subsequent generations unless
          similar samples are available from the parental generations (vide
          infra).
      4.  Not less than 20 females should be used for each mating, and not less
          than 25 individuals of F₁ and subsequent generations should be
          retained for observation and examination.
      5.  For some time prior to and during experiment the sheep of each genera-
          tion should be kept under the same conditions of management.
      6.  A uniform method of sampling should be adopted in all generations
          and duplicate or quadruplicate samples from each body area are
          desirable. The same technique should be applied in the examination
          of the samples.
      7.  Parental and all subsequent generations should be kept in the same
          locality so that variations due to climatic influence may be reduced
          to a minimum (vide infra).

That some criteria should be defined will become clear later, when it will be
shown that various workers have been unable to interpret their results at all
satisfactorily, chiefly because of the extreme variation in the parental types used.
In some experiments for degree of characterization the extremes of male and
female parental generations overlapped, and it obviously becomes difficult in such
cases to obtain significant figures in F₁ or subsequent generations. It is to be
feared that many of the results which are given later are useless from the point of
view of determining the genetic factors involved in the inheritance of fleece-
characters.

Since wool can be influenced in length, fineness, density, etc., by environment-
al agencies, standardized conditions are highly necessary for the experimental
flocks. The duration in time of wool-growth influences the ultimate length of
fibre in the sample. Since the first year's fleece may represent from 14 to 16
months' actual growth of wool (according to the relationship between the dates of
parturition and shearing), and since most lambs are born with a deciduous
birth-coat, which is replaced either partially or completely by the adult coat, and