320 



SCIENCE 



[N. S. Vol. XL. No. 1027 



this kind wMcIl have been sufSciently in- 

 vestigated it has been found that these dis- 

 tinctions — sometimes very great and affect- 

 ing most diverse features of organization — 

 are due to the presence or absence of ele- 

 ments, or factors, as we call them, which 

 are treated in heredity as separate entities. 

 These factors and their combinations pro- 

 duce the characteristics which we perceive. 

 No individual can acquire a particular 

 characteristic unless the requisite factors 

 entered into the composition of that indi- 

 vidual 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 magni- 

 tude is involved when we extend the appli- 

 cation of the same system to human char- 

 acteristics in general, yet the assumption 

 is one which I believe we are fully justified 

 in making. With little hesitation we can 

 now declare that the potentialities and apti- 

 tudes, physical as well as mental, sex, 

 colors, powers of work or invention, liabil- 

 ity to diseases, possible duration of life, 

 and the other features by which the mem- 

 bers of a mixed population differ from each 

 other, are determined from the moment of 

 fertilization; and by all that we know of 

 heredity in the forms of life with which we 

 can experiment we are compelled to believe 

 that these qualities are in the main dis- 

 tributed on a factorial system. By changes 

 in the outward conditions of life the ex- 

 pression of some of these powers and fea- 

 tures may be excited or restrained. For the 

 development of some an external oppor- 

 tunity is needed, and if that be withheld 

 the character is never seen, any more than 

 if the body be starved can the full height 

 be attained ; but such influences are super- 



ficial and do not alter the genetic consti- 

 tution. 



The factors which the individual receives 

 from his parents and no others are those 

 which he can transmit to his offspring ; and 

 if a factor was received from one parent 

 only, not more than half the offspring, on 

 an average, will inherit it. What is it that 

 has so long prevented mankind from dis- 

 covering such simple facts? Primarily 

 the circumstance that as man must have 

 two parents it is not possible quite easily to 

 detect the contributions of each. The indi- 

 vidual body is a double structure, whereas 

 the germ-cells are single. Two germ-cells 

 unite to produce each individual body, and 

 the ingredients they respectively contribute 

 interact in ways that leave the ultimate 

 product a medley in which it is difficult to 

 identify the several ingredients. When, 

 however, their effects are conspicuous the 

 task is by no means impossible. In part 

 also even physiologists have been blinded 

 by the survival of ancient and obscurantist 

 conceptions of the nature of man by which 

 they were discouraged from the application 

 of any rigorous analysis. Medical litera- 

 ture still abounds with traces of these 

 archaisms, and, indeed, it is only quite 

 recently that prominent horse-breeders 

 have come to see that the dam matters as 

 much as the sire. For them, though vast 

 pecuniary considerations were involved, 

 the old "homunculus" theory was good 

 enough. We were amazed at the notions of 

 genetic physiology which Professor Bald- 

 win Spencer encountered in his wonderful 

 researches among the natives of Central 

 Australia; but in truth, if we reflect that 

 these problems have engaged the attention 

 of civilized man for ages, the fact that he, 

 with all his powers of recording and deduc- 

 tion, failed to discover any part of the 

 Mendelian system is almost as amazing. 

 The popular notion that any parents can 



