no 



The Jcjurnal of Heredity 



This sort of work is \-er\' susceptible of 

 improvement through traininj^;, but they 

 were virtually just as much like each 

 other in the second test, i.e., their 

 ability to do puzzles which they had 

 never seen before. This is just what we 

 should expect if inborn causes were back 

 of the great mental similarity of twins; 

 but it docs not jjrove that heredity 

 and environment working together may 

 not have caused the striking resem- 

 blance. It does not separate heredity 

 from environment except on the specific 

 point of training in arithmetic. It 

 shows a failure of environment to make 

 itself manifest. 



Again, older twins and younger twins 

 were comi)ared . There were two groups — 

 one 9 to 11, and one 1 2 to 14 years of age. 

 The older twins showed no closer re- 

 semblance than the younger. The twins 

 would, through the inherent tendency 

 to diversification, tend a little towards 

 dissimilarity merely through the three 

 years of aging. They do, as a matter of 

 fact, become a little less similar during 

 the three years. 



This shows that there was nothing 

 in the identity of the home and school 

 life at that time to mold them to one 

 type. 



THIv MINU AND TWV. BODY 



Although we catmot much separate 

 heredity and environment specifically 

 by the use of present day data concern- 

 ing twins, they yet furnish a great deal 

 of evidence that is significant for mental 

 heredity. The fact is significant that 

 all the measurements show the similari- 

 ties to be on the average as strong for 

 the mental as for the jjhysical. The 

 fact that they are, for instance, just as 

 much alike in their ability to solve new 

 puzzles as they are in the color of their 

 eyes is very significant, since eye-color 

 differences are known through Men- 

 delian experiments to be foreshadowed 

 by diflerences in the germ-plasm. 



For actual measurements of the cfTtct 

 of a changing environment we must 

 keej) the heredity factor constant or 

 virtually so. In the efficiency experts' 

 experiment this rccjuircment was suffi- 

 ciently fulfilled. In the case cited of 

 the seed and the fertilizers, we are 



satisfied that we get a large enough 

 degree of identity in the seed by taking 

 a large enough number of seeds for 

 samples. In the same way htmian 

 environment walues could be tested by 

 using a large enough number of cases 

 to insure equality of germ-plasm in two 

 groujjs under difYering imposed environ- 

 ments. The careers of younger sons 

 and of older sons in the British nobility 

 and gentry ought to furnish material. 

 All the sons ought to average about the 

 same heredity, whether they are younger 

 or older. There is a good deal of dispute 

 on the question of the first born being 

 diff'erent from the others. The first 

 born ai:)pears to be more able, and also, 

 curiously enough, more unable. If he 

 is more apt to be a college professor he 

 is also more apt to be a criminal or a 

 high-grade imbecile. All this may be 

 a statistical fallacy or due to some- 

 thing surrounding the conditions of the 

 birth of a first born, survival values, etc. 

 The ])oint, so far as it concerns us, is 

 that it has to be taken into considera- 

 tion. But let us suppose that we 

 found very different mental character- 

 istics between second sons and third 

 sons: I would say that this might 

 be a good example of the power of 

 environment to modify traits. 



EFFECT OF SELECTION 



Some of it would be due to charactt rs 

 naturall\- fitting into their careers of 

 the army, the law or the church, but 

 some of the differences, if found, would 

 probably be due to the power of a 

 convention or habit of society to 

 jjroduce mental and moral differences. 

 We should have to follow out the careers 

 and know to what extent men actually 

 selected or stuck to congenial occui)a- 

 tions. If the church is only made u]) 

 of those men who find the life congenial 

 to their natures, then, of course, this 

 grouj) of sons will have different traits 

 from those who stick to the army. 

 The whole question is — and this must 

 always be thought of— is there a con- 

 straint imi)osed or is there not? In 

 most of the exi)erimtnts on animals a 

 ver\' sex'ere restraint on freedom of 

 choice is impf)S(.d. 



I was able to niaki- one experimental 



