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species oy means of experimental cvolutiona

Neither heredity nor environment is all-inclusive. Both are ne-
cessary for the highest form of development.

Heredity is the first determining force in constructive hygiene.

It may be defined as "the process which is responsible for the par-
ticular comtinetion of transmissable characters possessed by an or-
ganisme" (Burlingame)

Methods of Study of Heredity
Our knowledge of heredity has come to us by three principal methods:

a» Observational. "Like produces like" came as a result of mankind
comparing individuals, species, and races and observing certain
constant resemblances and differences.

bd. Statistical. This method studies characters singly and applies
quantitative methods to them. Francis Galton was its founder.
His researches were applied to several selected traits, as:
Genius, artistic ability, stature, eye color, and certain di-
seasese

The two principles connected with the name of Galton are:

(1) Law of Ancestral Inheritance: A statistical evaluation of
the contribution of each ancestor to the individual.

(2) Law of Filial Regression: Or tendency of children to return
to the average of the group.

The weakness of the method lies in the fact that it is not al-
ways possitle by observation alone to distinguish between in-
herited and environmental resemblances and differences. The
statistical method outlined the problem of heredity; it did not
solve it.

c. Experimental

(1) Mendel is the outstanding pioneer, with his study of the
breeding of pease His experiments enabled him to formulate
what are known as Mendel's Laws:

Law of segregation
Law of independent assortment

The principles of Mendelian inheritance apply to man. Many
inheritable traits, both normal and abnormal, have been
shown to be of this nature. See the list given in Storey's
Principles of Hygiene, Pp. 253-260.

(2) Later research has confirmed and extended Mendel's observa-
tions greatly. It has also added new conceptions which ex-
plain some facts which were apparently inconsistent with
his findings. The foremost of thesc newer principles are: