TWINS PROVE THE IMPORTANCE 



OF CHROMOSOMES 



The Most Powerful of All Forces Operating to Control Human Destiny — "The 



Most Important Things in the Whole Known World" — Are So Small 



That a Microscope Is Required to See Them 



Frederick Adams Woods 

 Lecturer on Biology, Massachusetts Institute of Technology 



WE KNOW at present that the 

 chief, though probably not 

 the sole, determiners of dilTer- 

 ences among higher animals 

 He in those infinitely important and 

 complex microscopic granules known as 

 chromosomes. The material out of 

 which they are formed remains con- 

 stant in every living cell, and they are 

 visible and subject to numerical count- 

 ing, at times of cell division. Most 

 people have never heard of chromo- 

 somes. But they will if they live long 

 enough ; unless, perchance, some other 

 word is introduced to take its place, 

 for chromosomes are among the most 

 important things, and, considering their 

 size, they are the most important things 

 in the whole known world. A genera- 

 tion from now the public will have 

 heard of chromosomes just as today 

 the public has heard of evolution and 

 of bacteria. Chromosomes and hered- 

 ity are in a way synonymous, but hered- 

 ity is used in various senses by different 

 people. When we say chromosomes, 

 we speak of things tangible, or at least, 

 if we cannot actually touch them, they 

 are objects that can be seen and meas- 

 ured and counted. 



Identical twins are among the most 

 interesting of beings, because they 

 alone have the same kind of chromo- 

 somes. This is due to the fact that 

 very early in embryonic life there 

 occurs an almost absolutely precise 

 division of the chromosomes, so that 

 two individuals are developed side by 

 side, each controlled by similar deter- 

 miners. The extreme resemblance of 

 identical twins is itself a proof of the 



importance of these determiners (chro- 

 mosomes), for otherwise we cannot 

 explain all the phenomena. 



It is known as a result of a great 

 number of observations that children 

 on the average resemble their fathers, 

 and their father's family, quite as much 

 as their mother's and their mother's 

 family. The far-reaching significance 

 of this every-day fact, like most won- 

 ders that are ever before our eyes, is 

 apt to be overlooked. Even this fact 

 alone is a proof of the extreme im- 

 portance of chromosomes, since the &gg 

 is many hundred times as large as the 

 sperm, but the chromosomes of the &gg 

 are no larger than those of the sperm. 

 They are practically the same size, and 

 presumably of corresponding chemical 

 structure and importance. 



Twins, then, are not needed to prove 

 the extreme importance of chromo- 

 somes. What twins prove is this : 

 They prove the lack of importance of 

 environment, but only in one aspect and 

 in one restricted meaning. 



There is a great deal of loose talk 

 about environment, all of which means 

 nothing and gets nowhere. The only 

 discussions that are today worth con- 

 sidering are those which are based upon 

 quantitative measurements. 



In a sense, environment is all-impor- 

 tant, for without nourishment, oxygen, 

 and warmth, growth is impossible ; and 

 without training, education, and social 

 control, the development of the higher 

 traits would not take place. But this 

 environment is customary and ex- 

 pected. The question of practical 

 interest is : How far do changes in the 



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