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.