August 28, 1914] 



SCIENCE 



291 



which animals and plants possess that we 

 feel justified in the expectation that with 

 continued analysis they will be proved to be 

 responsible for most if not all of the differ- 

 ences by which the varying individuals of 

 any species are distinguished from each 

 other. I will not assert that the greater 

 differences which characterize distinct spe- 

 cies are due generally to such independent 

 factors, but that is the conclusion to which 

 the available evidence points. All this is 

 now so well understood, and has been so 

 often demonstrated and expounded, that 

 details of evidence are now superfluous. 



But for the benefit of those who are un- 

 familiar with such work let me briefly 

 epitomize its main features and conse- 

 quences. 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 indi- 

 vidual 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 organ- 

 ism. We thus reach the essential principle, 

 that an organism can not pass on to off- 

 spring a factor which it did not itself re- 

 ceive in fertilization. Parents, therefore, 

 which are both destitute of a given factor 

 can only produce offspring equally desti- 

 tute 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 dis- 

 similar 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 vari- 

 ous matings can be predicted. 



Only those who have themselves wit- 

 nessed 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 dis- 

 integrate its features into the genetic ele- 

 ments by whose union the body was formed. 

 Set out in cold general phrases such dis- 

 coveries 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 segrega- 

 tion 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, youraelf — 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 contem- 

 plated 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 com- 

 pleteness after a cross — the merino sheep 

 or the f antail 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 segrega- 

 tion has been in reality imperfect. Of the 

 descent of quantitative characters we still 

 know practically nothing. These and hosts 

 of difficult cases remain almost untouched. 

 In particular the discovery of E. Baur, and 

 the evidence of Winkler in regard to his 

 "graft hybrids," both showing that the 



