38 BULLETIN 1121, U. S. DEPARTMENT OF AGRICULTURE. 
numerous factors are involved. In such cases inbred segregates, 
homozygous in all factors conducive to vigor would be rare and the 
degree of skewness would be imperceptible. 
It may be added that even if only one or two factors are involved, 
the skewness in F, would be imperceptible if environmental in- 
fluences play an important part in the variation. In the experi- 
ments on guinea pigs described in the present paper, over 90 per 
cent of the variation in such characters as size of htter and weight 
is demonstrably due to factors which are not genetic. No significant 
differences in skewness or even in variation can be found between 
inbreds and second generation crossbreds. But none is to be ex- 
pected under such conditions. | 
Finally, the success which some livestock breeders have had with 
close inbreeding and which Darwin obtained in at least one case 
with his morning glories indicates that inbred lines are produced 
occasionally which are thoroughly satisfactory from the standpoint 
of vigor. This point has been demonstrated most conclusively in 
the extensive experiments of the Wistar Institute conducted by 
Dr. Helen D. King (1918). In this experiment a strain of rats (two 
lines since the seventh generation) has been inbred, brother with 
sister, for 22 generations. Not only has full vigor been maintained 
but the inbreds have actually come to surpass the random-bred 
stock of the Wistar Institute in size and fertility. This result she 
attributes to careful selection. The strain of albino rats used was 
doubtless also rather homogeneous to begin with. The fact remains 
that long-continued, intensive inbreeding is not incompatible with 
a high degree of vigor.° 
It thus turns out that as far as the facts of inbreeding and cross- 
breeding are concerned the distinction between the hypotheses is 
largely one of wording. The choice between them depends on which 
involves the fewest unproved assumptions. So far as the writer 
knows, it has not been demonstrated in any specific case that a 
heterozygote may show an increase in vigor while the two homo- 
zygotes are indifferent. On the other hand, it has been noted 
repeatedly that there is a correlation among known Mendelian 
characters between dominance and vigor, or, looking at it from 
the other end, between recessiveness and deleterious effect. Collins 
(1921) prefers the latter form of statement as suggesting better the 
probable evolutionary significance of the phenomenon. 
Most of the mutations known in Drosophila are less vigorous than 
the normal strain (Morgan, Sturtevant, Muller, and Bridges, 1915)- 
They are also mostly recessive, at any rate as regards detrimental 
5 These experiments have also given a remarkable demonstration of the success of inbreeding associated 
with close selection, as a method of modifying a character so difficult to deal with as sex ratio. Two lines, 
separated in the seventh generation, were selected respectively for high and low ratio of males to females. 
An average sex ratio of about 122 became fixed in one line, about 82 in the other. 
