Page:Popular Science Monthly Volume 76.djvu/334

330 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 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 increased 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 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.

The direct researches which have essayed to separate the environment and heredity factors in the higher human traits, and measure