474 



The Journal of Heredity 



ssss 



c 



heritable variations presented by organ- 

 isms must be great, because it would be 

 impossible to have complex organs 

 evolving simultaneously, as they ob- 

 viously have done, unless the improve- 

 ments in the one did not modify or 

 interfere with improvements in the 

 others. If every variation making to- 

 ward the perfection of the eye were tied 

 lip with a variation in the ear, the 

 organs of digestion, and the structure 

 of the limbs, it seems inevitable that 

 there would be so much interference 

 with one another's progress that any 

 progressive evolution of a number of 

 complex organ systems would be prac- 

 tically impossible. Variations accumu- 

 lating toward the perfection of any one 

 organ, argues Weismann, would in all 

 probability, work toward the undoing 

 of various other organs. Independent 

 variability of parts must, therefore be 

 assumed in order to make the evolution 

 of a complex organization possible 

 through variation and natural selection. 



There is much apparent force in this 

 argument for the conception of the 

 organism as a mosaic product. Its 

 real weight is difficult to estimate, 

 plausible as it may appear, because we 

 know so little of the possibilities of 

 organismal variability. However the 

 assumption of any particular kind of 

 variability may increase or lighten the 

 task of explaining how evolution takes 

 place, it is obviously our first duty to 

 inquire whether or not organisms ac- 

 tually vary in the way alleged. Since 

 so much has been built upon the doctrine 

 of independent variability of parts, the 

 the burden of proof may fairly be held 

 to rest with those who espouse this 

 theory. 



Let us therefore consider some of the 

 alleged instances of independent varia- 

 tion. Take the classical case cited by 

 Weismann, of the small pit in the ear 

 which ran through several successive 

 generations. As this is an inherited 

 character, the germ plasm of the 

 person transmitting it must be slightly 

 different from that of a' person without 

 this defect. But does it follow that 

 "it can only depend ultimately on a 

 divergently constituted part of the 

 .germplasm, which can only affect this 



one spot on the head, and alter it, if it 

 is itself different from what is usual?" 

 If variations such as this could come 

 and go, leaving the rest of the organism 

 unmodified, we should be logically led, 

 I believe, to adopt Weismann 's con- 

 clusion that these variations depend on 

 independent carriers of some sort in the 

 germplasm. Weismann's reasoning is 

 good, so far as his doctrine of deter- 

 minants goes, if we grant his funda- 

 mental assumption. If a small pit in 

 the ear were absolutely the sole heredi- 

 tary difference between two human 

 beings we might be forced to consider it 

 as a unit character depending on a 

 special determinant, determiner, or other 

 germinal unit or entity. But do we 

 know that the facts are as Weismann 

 assumes ? 



STUDIES NOT DEEP ENOUGH 



I am quite sure that these people 

 with a pit in the ear have never been 

 very critically studied to find whether 

 or not this small character may not be a 

 mere expression of more general dif- 

 ferences in constitution. It might very 

 well be that this pit is simply a relatively 

 obvious manifestation of a very slight 

 difference which affects the organism as 

 a whole. The same may be true of the 

 white lock of hair and numerous other 

 characters which appear to vary in- 

 dependently of the rest of the body. 



The now neglected study of correlated 

 variability has revealed numerous cases 

 in which what appear as single varia- 

 tions have far-reaching connections. 

 Supernumerary horns in sheep are said 

 by Youatt to go along with great 

 "length and coarseness of the fleece." 

 In mammals in general there is a strong 

 tendency for variation to affect simul- 

 taneously hair, teeth and hoofs or 

 claws. Darwin points out that the 

 white star in the forehead of horses is 

 generally correlated with white feet, 

 and that in " white^rabbits and cattle, 

 dark marks often co-exist on the tips of 

 the ears and on the feet." |*oly- 

 dactylism; as is well known, tends to 

 affect both hands and feet. How are 

 we to interpret these correlations? 

 If hands and feet vary together do the 

 intervening parts of the skeleton re- 



