6
THE ESSEX NATURALIST.
have been the case, is a mystery; a mystery, however, which
would well repay solution, for it would certainly shed light
on one of the dark corners of evolution.
To my mind, however, the exceptional interest of asymmetry
as a biological problem lies in its relation to theories of growth
and inheritance. On the accepted chromosome theory of
heredity one must imagine that at one or more critical cell
divisions during the development of asymmetrical forms, some
sort of segregation of genes takes place which does not occur
in the more usual symmetrical species. But why, in the
normally asymmetrical species, should this always occur at the
same point or points? In the case of gynandromorphs it appears,
as a rule, to be simply an accident at what point in ontogeny
the sex chromosomes become separated for, as already men-
tioned, gynandromorphs may show almost any degree of
maleness or femaleness. Then as regards the "hormone"
theory of growth, which postulates the dependence of the
development of organs upon the stimulating effects of certain
secretions which, circulating in the blood, call forth specific
responses in the appropriate places, how is it possible to believe
that such substances can produce different effects on the two
sides of the body? These queries are not raised with any idea
of throwing doubt on the general correctness of the theories,
but simply to show how necessary it is that the facts of
asymmetry should be borne in mind in considering any theories
of growth and heredity.
The other morphological problem to which I propose to refer
is that of the geometrical correlation of the shapes of nearly
allied forms of animal life. In Professor D'Arcy Thompson's
most interesting and suggestive book on "Growth and Form"
there is a chapter "On the theory of transformations or the
comparison of related forms," in which the subject is dealt with
in a very attractive and illuminating way. The correlation is
brought about by what is called the method of co-ordinates which
may be briefly described as follows. Taking the outline drawing
of some animal or some part of an animal as our standard of
comparison, two series of parallel lines at equal distances apart
and at right angles to one another are drawn over it. These
are the co-ordinates, and they are necessarily intersected at
various points by the outline of the animal under consideration,