388 



The Journal of Heredity 



A SINGLE DIVIDED CELL PRODUCES 

 TWINS 



Now it sometimes happens — we do 

 not yet know why — that this original 

 cell, made from the two parent cells, 

 in one of its earliest divisions splits 

 entirely in two, and the two halves, 

 instead of remaining together as they 

 usually do, remain separate and grow 

 into separate individuals. In this case 

 the two new individuals will have ex- 

 actly the same heredity, and they are 

 the only human beings we know of who 

 have, and it is such individuals whom 

 we speak of as identical twins. They 

 are always of the same sex and always 

 resemble each other to an extraordinary 

 degree throughout the whole of their 

 lives. Fraternal twins, who do not re- 

 semble one another, and who may not 

 even be of the same sex, are the product 

 of two unions of two different mother 

 and father cells, and have simply hap- 

 jjcned to be born at the same time. 



There are cases where two of triplets 

 appear to be identical twins and the 

 third to have originated from a separate 

 cell. Cases of identical quadruplets are 

 theoretically possible by assuming that 

 the fertilized cell splits into four quar- 

 ters. If one of these failed to grow, 

 identical triplets would result. 



, IDENTICAL TWINS LIKENED TO 



DIVIDED PLANTS 



We have then, in these identical 

 twins, cases of human beings who 

 start out, not as two different mix- 

 tures of the free and variable cells 

 of their parents, but as halves, so to 

 speak, of one mixture. To roughly 

 illustrate the difference let us take two 

 apple seeds. They have come from the 

 same apple, but they will not grow into 

 apple trees producing the same kind of 

 apples. One may produce a tree which 

 has a very fair apple on it ; the other 

 may produce apples which are too sour 

 or bitter to eat. The trees themselves 

 may even be distinctly different ; one 

 may grow upright and the other be 

 spreading. The explanation is that they 

 have different heredities at the very 

 start — different pollen grains from a 

 flower on the same tree have fer- 



tilized different egg cells in the young 

 fruits. Supposing, however, we were to 

 divide a single seed from an apple and 

 that these two exact halves grew, 

 would we not then be able to produce 

 two apple trees which looked alike and 

 which, when they bore, had almost ex- 

 actly similar fruits on them? While it 

 is not possible to divide the seed, hori- 

 culturists have found that they get al- 

 most the same result by dividing the 

 plants after they are partly or entirely 

 grown up. The whole great fabric of 

 our cultivated varieties of fruit trees 

 is built up on this approximate identity 

 of the heredity of trees produced by 

 growing them from parts — buds — taken 

 from a single remarkable tree which 

 produces delicious fruits, and putting 

 them where they can grow and form 

 thousands of orchard trees. Millions of 

 navel orange trees, all of which pro- 

 duce fruits so much alike that even one 

 who is not an expert can tell they 

 are navel oranges at a glance, came 

 in this way from a single bud 

 which was put into a bitter-orange 

 tree in Riverside. Cal., in 1870. The 

 reason these all look so much alike, 

 even though grown in California or 

 Florida, is because their heredity at 

 the very start was so nearly identical. 

 They are not all ahsolutcly alike as they 

 are influenced by the climate and soil 

 in which they grow. The flavor of a 

 navel orange grown in Florida is dif- 

 ferent from one grown in California 

 and quite as different from one grown 

 in Bahia, Brazil, where the first buds 

 came from ; but they all have the navel 

 which is so characteristic of the variety 

 as to have given it its name. 



Roughly speaking, then, identical 

 twins can be compared with plants 

 which have been propagated by the 

 division of one individual. 



ALL DIFFERENCES NOT APPARENT 

 IN PHOTOGRAPHS 



There are certain questions which may 

 arise in the minds of those who look at 

 these photographs. I surmise these will 

 mainly deal with their invisible differ- 

 ences — differences in temperament, 

 mental ability, tastes, mechanical abili- 



