348 THE ZOOLOGIST. 



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 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 than if the body 

 be starved can the full height be attained ; but such influences are 

 superficial and do not alter the genetic constitution. 



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 discovering such simple facts ? Primarily 

 the circumstance that as man must have tioo parents it is not possible 

 quite easily to detect the contributions of each. The individual 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 literature 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 

 Baldwin 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 civilised man for ages, the 

 fact that he, with all his powers of recording and deduction, failed to 

 discover any part of the Mendelian system is almost as amazing. 

 The popular notion that any parents can have any kind of children 

 within the racial limits is contrary to all experience, yet we have 

 gravely entertained such ideas. As I have said elsewhere, the truth 

 might have been found out at any period in the world's history if 

 only pedigrees had been drawn the right way up. If, instead of 

 exhibiting the successive pairs of progenitors who have contributed 

 to the making of an ultimate individual, some one had had the idea 

 of setting out the posterity of a single ancestor who possessed a 

 marked feature such as the Habsburg lip, and showing the trans- 

 mission of this feature along some of the descending branches and 

 the permanent loss of the feature in collaterals, the essential truth 

 that heredity can be expressed in terms of presence and absence must 

 have at once become apparent. For the descendant is not, as he 

 appears in the conventional pedigree, a sort of pool into which each 



