HEREDITY BATESON. 363 



But for the benefit of those who are unfamiliar with such work let 

 me briefly epitomize its main features and consequences. Since 

 genetic factors are definite things, either present in or absent from 

 any germ cell, the individual may be either "pure bred" for any 

 particular factor, 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 fertilization to produce the parent organism. We thus 

 reach the essential principle, that an organism can not pass onto 

 offspring a factor which it did not itself receive in fertilization. 

 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 crossbred, which results from 

 the union of dissimilar germ cells, are mixed in character. Each 

 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 composition of the sev- 

 eral kinds of families formed from the various matings can be pre- 

 dicted. 



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 endeavor 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 bonds 

 of the known and the proved, you will feel something of that range of 

 insight into nature which Mendelism has begun to give. The ques- 

 tion 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 quantitative characters we still know practically nothing. 



