442 BOTANICAL GAZETTE [MAY 
the correctness of WEISMANN’Ss contention that “body and germ-cells are physio- 
logically distinct,”’ and that “body (or somatic) influences are not inherited.” 
The next six chapters deal with ‘“‘the greatest single discovery ever made in the 
field of heredity, Mendel’s law,” with illustrations drawn chiefly from the 
author’s experiments with guinea-pigs, rabbits, rats, and mice. After a careful 
statement of the general principles with concrete examples, attention is given 
to the determination of dominance, heterozygous characters and their “ fixa- 
tion,” atavism or reversion, evolution by loss or gain of characters, evolution 
of new races by variations in the potency of characters, modification of unit- 
characters by selection, and “blending” inheritance. This enumeration of the 
subjects treated suffices to indicate that the author’s discussion is no merely 
formal presentation of the ramifications of the Mendelian system. Instead, it . 
deals lucidly and entertainingly with a number of moot questions. It is pleas- 
ing to note that the author does not follow some other recent writers (BATESON, 
PUNNETT, DAVENPORT) in the view that dominance is always due to the pres- 
ence of a gene which is absent from the recessive form. He mentions several 
cases for which this explanation is not available, and leaves the problem as 
to the cause of dominance unsolved. He is convinced that unit-characters 
may be modified by selection. He says (p. 120): “In several cases I have 
observed characters at first feebly manifested gradually improve under selec- 
tion until they became established racial traits.’ While this must be accepted 
leading. The difficulty remains that selection can only pick out individuals 
already possessing the observed degree of development of any characteristic 
under consideration, and does not in any manner modify the qualities which 
will be possessed by the offspring of the selected individuals. It only permits 
have the character in question. Under “blending inheritance” the now well- 
known case of skull-size and ear-length in rabbits is discussed, and the view 
is expressed that in the light of experimental results of NitssoN-EHLE, East, 
and others, such cases of apparent blending may really prove to be segregating 
— in which a considerable number of units are involved. 
ap. ix the effects of in-breeding are considered, and the reviewer’s 
— is indorsed, that the apparent deterioration is generally due to the 
formation of homozygous strains, whereby the stimulus is lost which comes 
from the “bringing together of differentiated gametes, which, reacting on each 
other, produce greater metabolic activity.”’ In this connection the statement 
is made (p. 150) that “under self-pollination for one generation following a 
cross, half the offspring become homozygous; after two generations three- 
fourths of the offspring are homozygous’’; and so on. This statement is 
misleading as it stands, and is literally true only in the case of monohybrids. 
A second cause recognized for deterioration following in-breeding is the ap- 
pearance of recessive defects, such as albinism, etc., a cause which has been 
specifically pointed out by DAVENPORT. 
