44Section I.V. G. Ruggeri.
Hurst, also, has shown the hereditary character of the colour of the iris,
availing himself of a slightly different distinction(1). He has made two cate
gories of colours of the iris—simple irises and double irises. The first are
blue or blue-grey; the double, which include all the others, have more or
less of pigment in those layers in which the simple irises are, on the other
hand, destitute of pigment. Now, the double irises are dominant over the
simple, so that they can be homozygote or heterozygote, and this cannot be
recognised externally, but from what we have previously proved it can be
shown what eventualities are possible, and with what results. Thus Hurst
has easily proceeded to the respective groupings of the progenitors, obtaining
that view which confirms the Mendelian laws.
Colour of the Iris of the Children.
Colour of the Iris of the Parents.Total No.
of Cases.Double.Single.Double : single (report
for 4) found.Proportion.
Single x Single ...IOIOIOIO: 40 : 4
Double X Double omoz.195195O4: 04 : O
Double heters. X Double6345l82-86: 1,143:1(4 0*22
heterz. ...
Single x Double omoz. ...6666O4: 04 : 0
Single X Double heterz.2581371212,12: 1,8822 ( + 0-13)
Finally, Carlo Davenport has established the order of dominance by the
form of the hair(2), which also obeys the Mendelian law. Straight or smooth
hair, like that of a horse, is recessive with regard to curly or woolly hair,
so that parents with smooth hair can only have children with smooth hair.
And, therefore, this kind of hair has a homogeneous and compact diffusion,
as is seen in the Chinese and in less degree in the native Americans. But
where these smooth-haired races meet with curly-haired races these can only
maintain the smooth form in a fraction of the descendants, which fraction,
25 per cent., shows atavism, as we have seen. In the same time, there
fore, the percentage of curly hair does not increase compared with that of
smooth hair, but is maintained in 25 per cent, of the descendants. The
other two quarters, which usually would be conjoined to the dominant char
acter, i.e., to the curly variety, in this case are modified, passing to the
wavy type. The dominance of the curly character is incomplete.
In the Philippine Islands, according to the recent researches of Bean,
the dominance of the curly character, which is here presented by the Negrito
type, has been confirmed. Whence many individuals may at first sight be
mistaken for Negritos, while these are hybrids who present no Negrito
(1) C. C. Hurst, On the Inheritance of Eye-Colour in Man, Pro. Royal Soc. B., LXXX.
(1908), p. 85.
(2) C. B. Davenport, Heredity of some human physical characters. Pro. Soc. for
Experitn. Biol, and Med. (1908), p. 101.
