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 



