Although far less noticed, a companion discovery was made by Charlotte Auerbach and William
Robs on at the Unive rs ity of Edinburgh in the early 1940s. Working with mus tard gas , they found
that this chemical produces permane nt chromosome abnormalities that cannot be
dis tinguis hed from thos e induced by radiation. Tes ted on the f ruit fly, the s ame organism
Muller had us ed in his original work with X-rays , mus tard gas als o produced mutations. Thus
the first chemical muta g en was discovered. Mus tard gas as a mutagen has now been joined by
a long list of other chemicals known to alter genetic material in plants and animals. To
unde rs tand how che micals can alter the cours e of heredity, we mus t firs t watch the bas ic
dra ma of life as it is played on the stage of the living cell. The cells compos ing the tiss ues and
organs of the body mus t have the powe r to increas e in number if the body is to grow and if the
stream of life is to be kept flowing from generation to generation. This is accomplis hed by the
proces s of mitosis, or nuclear division. In a cell that is about to divide, changes of the utmost
importance occur, firs t within the nucleus , but e ventually involving the e ntire cell. Within the
nucleus , the chromos omes mys terious ly move and divide, ranging themselves in age-ol d
patterns that will s erve to dis tribute the de terminers of heredity, the genes , to the daughter
cells. Firs t they as s ume the form of elongated threads , on which the genes are aligned, like
beads on a s tring. Then each chromos ome divides lengthwis e (the genes dividing als o). When
the cell divides into two, half of each goes to each of the daughter cells. In this way each new
cell will contain a complete set of chromosomes, and all the genetic information enc oded
within them. I n this way the integrity of the race and of the s pecies is pres erved; in this way like
begets like.
A special kind of cell division occurs in the formation of the germ cells. Because the
chro mos o me n umber f or a given s pecies is cons tant, the egg and the s perm, which are to unite
to form a new individual, mus t carry to their union only half the s pecies numbe r. This is
accomplis hed with extraordinary precis ion by a change in the behavior of the chromos omes
that occurs at one of the divisions producing thos e cells. At this time the chromos omes do not
s plit, but one whole chromos ome of each pair goes into each daughter cell.
In this elemental drama all life is revealed as one. The events of the process of cell division are
common to all earthly life; neithe r man nor a moeba, the giant s equoia nor the s imple yeas t cell
can long exis t without carrying on this proces s of cell division. Anythi ng that dis turbs mitos is is
theref ore a grave threat to the welfare of the organism affected and to its des cendants.
‘The major features of cellular organization, including, for instance, mitosis, must be much older
than 500 million years—more nearly 1000 million,’ wrote George Gaylord Simpson and his
colleagues Pittendrigh and Tiffany in their broadly encompas s ing book entitled Life. ‘In this
sense the world of life, while surely fragile and complex, is incredibly durable through ti me—
more durable than mountains. This durability is wholly dependent on the almos t incredible
accuracy with which the inherited information is copied from generation to generati on.’
But in all the thous and million years envis ioned by thes e authors no threat has s truck s o
directly and so forcefully at that ‘incredible accuracy’ as the mid-20th century threat of man-
made radiation and man- ma de and man-disseminated chemicals. Sir Macfarlane Burnet, a
dis tinguis hed Aus tralian phys ician and a Nobel Prize wi nne r, cons iders it ‘one of the mos t
significant medical features’ of our time that, ‘as a by-product of more and more powerf ul
therape utic procedures and the production of che mical s ubs tances outs ide of biological
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