80 'WHAT IS A SPECIES?' 



was never seriously undertaken, — and a few years later 

 Darwin added the following postscript to a letter to 

 Huxley, January 7, [1867]. 



' P.S. — Nature never made species mutually sterile by 

 selection, nor will men.' x 



This was probably only an off-hand expression of 

 opinion, not intended to be taken seriously. An alto- 

 gether hopeless attitude would not be reasonable until 

 the suggested scheme had been applied many times, 

 and in several parts of the animal and vegetable king- 

 doms. 



But the positive results demanded by Huxley, even if 

 obtained, would by no means justify his far-reaching con- 

 clusions. If the barrier of sterility were thus artificially 

 produced, we should be very far from the proof that its 

 existence in nature is due to the same kind of cause, 

 viz. selection. If Darwin was right in his controversy 

 with Wallace, if ' Nature never made species mutually 

 sterile by selection', the suggested experiment would 

 merely do by Artificial Selection what is not done by 

 Natural Selection. 



Interspecific Sterility an incidental consequence of 

 Asyngamy. 



It is by no means difficult to understand the mutual 

 sterility which is usual between natural species as an inci- 

 dental result of their separation by Asyngamy for a long 

 period of time. In the process of fertilization a portion 

 of a single cell nucleus from one individual fuses with 

 a portion from another individual, the two combining 

 to form the complete nucleus of the first cell of the 

 offspring, from which all the countless cells of the future 

 individual will arise by division. Each part-nucleus con- 

 tains the whole of the hereditary qualities received from 

 and through its respective parent, and must therefore be 

 of inconceivable complexity. We can only speak in 

 generalities about processes of which so little is known, 

 but we cannot be wrong in assuming that sterility is 

 sometimes due to the fact that the complex architecture 

 1 More Letters, vol. i, p. 277, Letter 197. 



