8 PRESIDENT’S ADDRESS. 
or its absence, if he is constituted by the union of two germ-cells both 
possessing or both destitute of that factor. If the individual is thus 
pure, all his germ-cells will in that respect be identical, for they are 
simply bits of the similar germ-cells which united in fertilisation to 
produce the parent organism. We thus reach the essential principle, 
that an organism cannot pass on to offspring a factor which it did not 
itself receive in fertilisation. Parents, therefore, which are both 
destitute of a given factor can only produce offspring equally destitute 
of it; and, on the contrary, parents both pure-bred for the presence 
of a factor produce offspring equally pure-bred for its presence. 
Whereas the germ-cells of the pure-bred are all alike, those of the 
cross-bred, which results from the union of dissimilar germ-cells, are 
mixed in character. Hach positive factor segregates from its negative 
opposite, so that some germ-cells carry the factor and some do not. 
Once the factors have been identified by their effects, the average com- 
position of the several kinds of families formed from the various 
matings can be predicted. 
Only those who have themselves witnessed the fixed operations of 
these simple rules can feel their full significance. We come to look 
behind the simulacrum of the individual body, and we endeavour to 
disintegrate its features into the genetic elements by whose union the 
body was formed. Set out in cold general phrases such discoveries 
may seem remote from ordinary life. Become familiar with them and 
you will find your outlook on the world has changed. Watch the effects 
of segregation among the living things with which you have to do— 
plants, fowls, dogs, horses, that mixed concourse of humanity we call 
the English race, your friends’ children, your own children, yourself— 
and however firmly imagination be restrained to the bounds of the 
known and the proved, you will feel something of that range of insight 
into Nature which Mendelism has begun to give. The question is 
often asked whether there are not also in operation systems of descent 
quite other than those contemplated by the Mendelian rules. I myself 
have expected such discoveries, but hitherto none have been plainly 
demonstrated. It is true we are often puzzled by the failure of a 
parental type to reappear in its completeness after a cross—the merino 
sheep or the fantail pigeon, for example. These exceptions may still 
be plausibly ascribed to the interference of a multitude of factors, a 
suggestion not easy to disprove; though it seems to me equally likely 
that segregation has been in reality imperfect. Of the descent of quan- 
titative characters we still know practically nothing. . These and hosts 
of difficult cases remain almost untouched. In particular the discovery 
of KH. Baur, and the evidence of Winkler in regard to his ‘ graft hybrids,’ 
both showing that the sub-epidermal layer of a plant—the layer from 
which the germ-cells are derived—may bear exclusively the characters 
