Variation as an Organic Function 173 
characterise the germ-cell and the soma are manifestly quite different. 
The postulation of • parallel degrees of stability in germinal and 
somatic morphological characters is of no value whatever in this 
connection. What is implied by the theory of spontaneous variation 
is that those physiological reactions which are the most important 
factors in the equilibrium of the germ-cell, and therefore presumably 
the most stable, are always correlated with the most important and 
stable somatic structures and functions; or, to put it in a different way, 
that any changes in the germinal physiological reaction-complex which 
are mutually adjusted with respect to the functioning of the cell, 
and which affect the more important somatic characters, will do so 
in a way that is also mutually compensative with respect to somatic 
function. If the germinal and somatic cycles are functionally inde¬ 
pendent the probability of this happening in any single instance is 
very small; that such a parallelism would be maintained through a 
long succession of spontaneous germinal variations is too remote to 
be worth consideration. 
A single unfavourable mutation, if sufficiently large, would at 
once give rise to a non-viable type, and if a truly specific and not an 
accidental individual phenomenon, would lead to the ultimate ex¬ 
tinction of the species. The same result would occur sooner or later 
through an unfavourable combination of smaller mutations or a 
series of minute harmful changes in the same direction. The advocates 
of orthogenesis have indeed frankly admitted such a possibility, as 
for instance Duerden’s suggestion that there is a progressive diminu¬ 
tion in the toes of the African ostrich, which will lead to its ultimate 
extinction. A similar progressive change in a more vital organ, such 
as the cardiac valves, unless correlated with other compensative 
variations in the structure of the heart and the circulatory system, 
would attain the same result much more speedily. But, as we have 
seen, if germinal changes are spontaneous, there is no reason why 
they should be mutually compensative in relation to somatic func¬ 
tion. There is, of course, the possibility that the soma itself is capable 
of effecting some readjustment during ontogeny; but this must 
necessarily be very limited or otherwise the facts of heredity would 
be meaningless. 
We have endeavoured to show in the above discussion that the 
continued hereditary transmission of the physiologically important 
structures and functions through long phylogenies implies that there 
is some kind of functional interaction between the germinal and 
somatic cycles as a result of which germinal changes, at least such 
