33 o THE POPULAR SCIENCE MONTHLY 



quently twins differ much in size at the time of their birth, even when 

 they are afterwards known as " identical twins " and are difficult to 

 distinguish apart. The puny twin must have been at a disadvantage 

 during uterine life, but this has no permanent effect and is all made 

 up in the end. 



It is well known that lecithin given in small quantities in the food 

 will increase the rate of growth of mammals. But it has not been 

 shown, as far as I know, that the ultimate sizes of the adults are 

 thereby made to vary. Let us see what proportionate change in rate 

 of growth has been effected by experiments of this sort . 



The communications of Hatai 41 shows that guinea-pigs, rabbits, 

 dogs and rats, after from one to two months' treatment with lecithin, 

 have their rate of growth so altered that the amount of growth is in- 

 creased from 1.29 to 4.60 times the normal growth during this period. 

 The total weight of the animals is, however, but slightly increased. 

 The animals are merely rendered about two to five per cent, heavier 

 than they normally would have been during the same period of time. 

 Hatai, in referring to the experiments of Danilewski on the eggs of 

 frogs, states that frogs' eggs placed in water containing 1 to 1,500 by 

 weight of lecithin, gained in fifty-four days, 300 per cent, more in 

 weight than those reared in ordinary water. 



This would give one the impression that the changes were about the 

 same in ratio for mammals as for tadpoles since the amount of actual 

 additional growth in mammals may be fully three times as much with 

 lecithin as without it. On looking up Danilewski's 42 original paper 

 I find, first that the proportion of lecithin used was not 1 to 1,500 but 

 1 to 15,000 for the frogs; and second, what is more important, that. the 

 gain of 300 per cent, refers not to the portion gained during the interval 

 when lecithin was given, but to the total weight of the organisms. 

 Thus tadpoles may be made to vary 300 per cent, of their total weight, 

 mammals about two to five per cent. At the same time it seems that 

 the tadpoles are nearly doubled in linear dimensions. It is evident 

 that the mammals are but slightly altered in linear dimensions. 



Thus the experimental work of biologists indicates, when we take a 

 comparative bird's-eye view of modification, that environment will be 

 found to be working upon human brain and nerve tissue at its minimum 

 of efficaciousness. Let us see what direct statistical experience has to 

 say on this important problem. 



Mental and Moral Traits 

 The direct researches which have essayed to separate the environ- 

 ment and heredity factors in the higher human traits, and measure 



41 " The Effect of Lecithin on the Growth of the White Rat," Amer. Journal 

 of Physiology, 1904. 



43 Comptes Rendus, CXXI., 1895. 



