May 8, 1914] 



SCIENCE 



687 



Professor Eamaley's note might lead one to 

 infer that since Dr. MacDowell's work was 

 carried out under my supervision and since 

 his paper has been published with my approval, 

 therefore I (in common with Mendelians 

 generally) share the views expressed concern- 

 ing Mendelian factors in size inheritance, but 

 this is not entirely true, and to avoid further 

 possible misunderstanding I write this note. 

 Dr. MacDowell's observations I believe to be 

 accurate; they were made with great care and 

 were checked in every possible way. I have 

 kept in close touch with his work at every 

 stage of its progress and have found it un- 

 impeachable. Few investigators with whom 

 I have been associated have shown such apti- 

 tude for exact and critical work as he dis- 

 played from the beginning. I endorse his 

 observations fully. 



But the facts observed are capable of differ- 

 ent theoretical interpretations. In regard to 

 these I have encouraged in Dr. MacDowell the 

 fullest freedom of choice. He has adopted one 

 for which much can be said, that of multiple 

 Mendelian factors, which at times has ap- 

 pealed to me strongly, and the argument for 

 which I have presented elsewhere (" Heredity," 

 D. Appleton & Co., 1911) at some length. 

 This theory has also been developed inde- 

 pendently by Lang (1910), East (1910), Emer- 

 son (1910) and others. It accounts for the 

 facts fully if certain basic assumptions are 

 allowed, about which, however, I am growing 

 more skeptical the more closely I examine 

 them. Dr. MacDowell, in the passage quoted 

 by Professor Ramaley, truthfully says of this 

 theory, " It goes hand in hand with the muta- 

 tion and pure-line doctrines of De Vries and 

 Johannsen." But suppose one is not pre- 

 pared to accept those doctrines, what then be- 

 comes of the multiple factor hypothesis? It 

 is left without adequate basis. If the multiple 

 factor hypothesis must stand or fall with the 

 pure-line doctrine, I for one can not accept it, 

 for the foundations of the pure-line doctrine 

 appear to me very insecure. 



What in brief are the facts regarding size 

 inheritance which call for explanation? 



Fortunately, observers are quite in agreement 

 concerning them. 



1. Occasionally an unmistakable Mendeliz- 

 ing factor is concerned in size inheritance. 

 One was discovered by Mendel himseK (1866) 

 and its existence has been repeatedly verified, 

 namely, the differential factor between tall and 

 dwarf races of garden-x)eas. Tall and dwarf 

 conditions in other plants behave in a similar 

 way, that is as Mendelian alternative condi- 

 tions showing both dominance and segregation 

 in crosses. Brachydactyly in man is a varia- 

 tion like dwarfness in plants, in which the 

 growth habit is altered, the skeleton being 

 abnormally short and compact throughout. 

 This character is a Mendelian dominant 

 (Farabee, 1905, Drinkwater, 1908). The 

 shorter, more compact form of Dexter cattle, in 

 contrast with the Kerry breed, is a Mendelian 

 character (Jas. Wilson, 1909) probably similar 

 in nature. Doubtless the same was true of 

 the short-legged Ancon sheep mentioned by 

 Darwin (1878, "Animals and Plants"). 



From the mere fact that a Mendelian factor 

 may be involved in a size difference, it by no 

 means follows that all size differences are due 

 to Mendelian factors. Such Mendelizing 

 factors affecting size as have just been enu- 

 merated are distinctly rare. They are not 

 discoverable at all in the cases studied by 

 MacDowell, which involve neither dominance 

 nor segregation in a 1:2:1 ratio. Even in 

 cases involving an unmistakable Mendelian 

 factor, as the tall-dwarf cross in peas, it is not 

 to be supposed that no other factors affect 

 size. For are all dwarf peas of the same 

 height, or are all tall peas of the same height? 

 No, there are differences among each sort, 

 differences which are heritable also, since one 

 dwarf variety differs from another in its mean 

 height. 



2. Ordinary differences in size (such as do 

 not involve a change in the growth habit) 

 among animals or plants do not Mendelize in 

 the ordinary acceptation of the term. When 

 races are crossed which differ widely in size, 

 the first filial (F,) generation is intermediate 

 between the parents and often not more varia- 

 ble than one of the parent races. But the 



