394 
NATURE 
[Marcu 24, 1923 

west ; but the slickensides show that the movement 
was oblique, partly down the fault planes and partly 
parallel to the strike of the faults to the north-west, 
the latter component being the more important, so 
that the total movement to the west must have been 
considerable. At the same time there has been a 
general tilt of the country in that direction. Iam given 
to understand that somewhat similar faulting occurs 
in the South of Ireland, and, no doubt, it exists else- 
where on the eastern shores of the Atlantic. The total 
westward movement visible on the land does not 
amount to more than a few miles, but the downward 
displacement that accompanied it must have resulted 
in the submergence of a large area to the west, and the 
same structures, no doubt, extend still farther in that 
direction under the sea. In the actual neighbourhood 
of the rift (to use the convenient term employed by 
Prof. J. W. Gregory) there may well be a complete 
absence of the Sial, so that the Sima would be found 
close below the abysmal deposits ‘of the deep sea, as 
Wegener supposed to be the case. There is, however, 
no reason to suppose that the opposite shores of the 
continents represent the actual margins of the rift, 
and we cannot expect to find the close correspond- 
ence between them which he endeavours unsuccess- 
fully to demonstrate. Submarine plateaus rising in 
the midst of greater depths may represent portions 
of Sial submerged between two rifts. 
The solution of these problems should be regarded 
as an urgent task for the immediate future. There is 
little doubt that further information with regard to 
the density of rocks below the sea-bottom would result 
from the systematic measurement of the variation of 
gravity at sea with concurrent determination of its 
depth. The new methods that are now available are 
at once more rapid and more trustworthy than those 
previously employed, and might well be carried out 
either by the Navy or the great ocean liners. Valuable 
information, too, will be yielded by the E6étvés 
balance with respect to the rocks below the sea in the 
immediate neighbourhood of the shore. 
Joun W. Evans. 
The Function of Mendelian Genes. 
In Nature of March 3 there appears a letter from 
Mr. Julian Huxley on ‘ The Function of Mendelian 
Genes,”’ in which he criticises a paragraph in a review 
of mine published in Nature of January 20. As I 
think that the difference between Mr. Huxley and 
myself is due to a certain extent to a misunderstanding 
of my meaning, perhaps you will allow me space to 
make a brief reply. 
Mr. Huxley’s main point is that in treating 
Mendelian genes as measures of pathological damage 
to the hereditary substance, I forgot that each dis- 
covery of a presumably pathological mutant gene 
implied the existence of an allelomorphic normal 
gene in the type, and that in this way we were 
enabled to analyse the hereditary machinery of the 
type. 
The paragraph to which Mr. Huxley alludes was a 
small item in a review devoted to vitalism. Mr. 
Huxley and I had a prolonged battle in Science 
Progress last year, and perhaps before long we may 
have another friendly encounter in the same journal. 
As he alludes to this contest in a footnote, I may 
here say that he is incorrect in stating that he forced 
me to admit that not all mutations were pathological. 
All I said was that I could not make such a universal 
statement without examining each case, but I may 
add that I have yet to meet with the Mendelian 
mutation which is not pathological. My answer to 
Mr. Huxley is that of course I recognise the existence 
of hypothetical allelomorphic normal genes, which 
NO. 2786, VOL. 111] 


taken together make up the hereditary machinery of 
the type, but I doubt the value of the analysis of 
this machinery into genes at all. The only analysis 
of the hereditary complex which seems to me to be 
at all interesting or fruitful, is its dissociation into 
the factors out of which it was actually historically 
built up. I regard this complex as the solidification, 
so to speak, of the reactions of the race to the varying 
experiences through which they have gone during past 
ages. New habits have been superposed on old ones, 
with accompanying modifications of structure; and 
when we have unravelled this history completely, we 
have given as exhaustive an account of the origin of 
the hereditary machinery as is possible. 
The normal “ gene ”’ is an imaginary section of this 
machinery invented to account for the damage which 
a mutant gene introduces. Mr. Huxley alludes to 
the existence of multiple allelomorphs as proving that 
the recessive mutant gene is not the mere absence 
of something which we call the dominant gene. I 
think that a series of multiple allelomorphs inevitably 
suggests a graded series of varying degrees of damage, 
or, aS we may phrase it, a series of increasing intensities 
of defect. Such a series is given by the mutants of 
the red eye of the wild Drosophila. These are listed 
as vermilion, scarlet, cherry, pink, eosin, cream, and 
white! What other plausible explanation can be 
given of these than the gradual disappearance of the 
dark red pigment of the normal eye ? 
One of the mutant genes of Drosophila produces a 
variation termed “‘ balloon wing.’’ In insects showing 
this variation the two layers of ectoderm forming the 
wing are widely divaricated from each other, the 
space between them being occupied by a bubble of 
air. Now there is a general consensus of opinion 
based on paleontology, embryology, and comparative 
anatomy as to the evolutionary history of insects’ 
wings. They began as slight lateral extensions of 
the dorsal terga of the thorax, at first in all three seg- 
ments ; but later they were confined to the posterior 
two segments. In the beginning they served merely 
as parachute planes to break the fall of the insect 
when it leaped into the air; later, as they grew longer 
and flexible, they became capable of independent 
movement, and so developed into the varied types of 
wing found at the present day. On what phase, one 
may ask, of this history of progressive functional 
evolution does the existence of the balloon wing 
mutant throw the smallest light ? 
We are gradually learning to recognise that the 
body of an animal is built up by the co-operation of 
the semi-independent growths of a number of tissues 
and organs which, however, mutually limit and deter- 
mine the extent of each other’s growth. The com- 
promise which is arrived at, is expressed in the normal 
specific or racial structure of the animal, and may be 
expressed by the term ‘“‘regulatory balance.” When 
the race is exposed to new surroundings, the regulatory 
balance is altered and a new race is evolved. This 
accounts for the fact noticed by Sturtevant that 
allied species differ from one another in numbers of 
minute points affecting all the organs of the body, 
whereas mutations are characterised by marked dif- 
ferences affecting only one or two organs. Mutations 
may be defined as pathological disturbances of this 
regulatory balance; if they are so severe as to pro- 
duce a noticeable effect on the offspring when intro- 
duced by only one parent, they are dominant; if 
their effects are only apparent when both parents are 
affected, they are recessive. 
Mr. Huxley’s comparison of the mutant black 
mouse to the melanic local races of wild species is 
unfortunate. The black mouse (which I have often 
reared) is covered with a fur of so uniform colour as 
to make it exceptional among mammals. It may be 
—— 
