HEREDITY BATESOISr. 377 



conclusion I drew was the negative one, that, though we must hold 

 to our faith in the evolution of species, there is little evidence as to 

 how it has come about and no clear proof that the process is con- 

 tinuing in any considerable degree at the present time. The thought 

 uppermost in our minds is that knowledge of the nature of life is 

 altogether too slender to warrant speculation on these fundamental 

 subjects. Did we presume to offer such speculations they would have 

 no more value than those which alchemists might have made as to 

 the nature of the elements. But though in regard to these theoretical 

 aspects we must confess to such deep ignorance, enough has been 

 learned of the general course of heredity within a single species to 

 justify many practical conclusions which can not in the main be 

 shaken. I propose now to develop some of these conclusions in re- 

 gard to our own species — man. 



In my former address I mentioned the condition of certain animals 

 and plants which are what we call " polj^morphic." Their popula- 

 tions consist of individuals of many types, though they breed freely 

 together with perfect fertility. In cases of this kind which have been 

 sufficiently investigated it has been found that these distinctions — 

 sometimes very great and affecting most diverse features of organiza- 

 tion — are due to the presence or absence of elements, or factors, as 

 we call them, which are treated in heredity as separate entities. 

 These factors and their combinations produce the characteristics 

 Avhich we perceive. No individual can acquire a particular charac- 

 teristic unless the requisite factors entered into the composition of 

 that individual at fertilization, being received either from the father 

 or from the mother, or from both, and consequently no individual can 

 pass on to his offspring positive characters which he does not himself 

 possess. Rules of this kind have already been traced in operation in 

 the human species, and though I admit that an assumption of some 

 magnitude is involved when we extend the application of the same 

 system to human characteristics in general, yet the assumption is one 

 which I believe we are fully justified in making. With little hesita- 

 tion we can now declare that the potentialities and aptitudes, physical 

 as well as mental, sex, colors, powers of work or invention, liability 

 to diseases, possible duration of life, and the other features by which 

 the members of a mixed population differ from each other, are deter- 

 mined from the moment of fertilization, and by all that we know of 

 heredit}' in the forms of life with which we can experiment we are 

 compelled to believe that these qualities are in the main distributed on 

 a factorial system. By changes in the outward conditions of. life the 

 expression of some of these powers and features may be excited or 

 restrained. For the development of some an external opportunity is 

 needed, and if that be withheld the character is never seen any more 



