Sterility of Hybrids 97 
rechten Weg zur Ergriindung der Gravitationsgesetze fiihrte.” We 
cannot pretend that the words are not still true, but in Mendelian 
analysis the seeds of that apple-tree at last are sown. 
If we were asked what discovery would do most to forward our 
inquiry, what one bit of knowledge would more than any other 
illuminate the problem, I think we may give the answer without 
hesitation. The greatest advance that we can foresee will be made 
when it is found possible to connect the geometrical phenomena 
of development with the chemical. The geometrical symmetry of 
living things is the key to a knowledge of their regularity, and 
the forces which cause it. In the symmetry of the dividing cell 
the basis of that resemblance we call Heredity is contained. To 
imitate the morphological phenomena of life we have to devise a 
system which can divide. It must be able to divide, and to segment 
as—grossly—a vibrating plate or rod does, or as an icicle can do as it 
becomes ribbed in a continuous stream of water; but with this dis- 
tinction, that the distribution of chemical differences and properties 
must simultaneously be decided and disposed in orderly relation to 
the pattern of the segmentation. Even if a model which would do 
this could be constructed it might prove to be a useful beginning. 
This may be looking too far ahead. If we had to choose some one 
piece of more proximate knowledge which we would more especially 
like to acquire, I suppose we should ask for the secret of interracial 
sterility. Nothing has yet been discovered to remove the grave 
difficulty, by which Huxley in particular was so much oppressed, that 
among the many varieties produced under domestication—which we 
all regard as analogous to the species seen in nature—no clear case 
of interracial sterility has been demonstrated. The phenomenon is 
probably the only one to which the domesticated products seem to 
afford no parallel. No solution of the difficulty can be offered which 
has positive value, but it is perhaps worth considering the facts in 
the light of modern ideas. It should be observed that we are not 
discussing incompatibility of two species to produce offspring (a totally 
distinct phenomenon), but the sterility of the offspring which many 
of them do produce. 
When two species, both perfectly fertile severally, produce on 
crossing a sterile progeny, there is a presumption that the sterility 
‘is due to the development in the hybrid of some substance which can 
only be formed by the meeting of two complementary factors. That 
some such account is correct in essence may be inferred from the 
well-known observation that if the hybrid is not totally sterile but 
only partially so, and thus is able to form some good germ-cells 
which develop into new individuals, the sterility of these daughter- 
individuals is sensibly reduced or may be entirely absent. The 
D. 7 
