STERILITY OF HYBRIDS. 425 



must be sterile. The above remark, that there is but 

 one reproductive point in that individual, has reference 

 solely to the animals highest in the scale of develop- 

 ment. A plant has many such reproductive cen- 

 ters, and so. have many animals low in the scale of de- 

 velopment. But, mutatis mutandis, the principle also 

 obtains with them. Strictly stated, it should be said, 

 that in a hybrid, the influence of the aggregate deter- 

 mines to a point or points other than those devoted to 

 purposes of exudation. 



The reason, why closely-allied species produce at 

 times hybrid offspring, is because they possess organs 

 and features somewhat alike ; and their reproductive 

 forces, having come, severally, from pure species, need 

 only to be sufficiently isochronous, to coalesce and 

 concur in the mutual formation of an organism. The 

 question, with the reproductive elements, in this 

 case, is not one of formative power ; for each, having 

 been 'derived from a pure species which concentrated 

 the influence of its aggregate upon the tissue which 

 was physically adapted for exudation, they have, other 

 things equal, the power of integration. The question 

 is one, simply of the possibility of their uniting and 

 working to the same end. The reason, why very dis- 

 tinct species are absolutely sterile with each other, 

 — not capable even of the formation of a hybrid, — is not 

 because of the reproductive elements of the two lack- 

 ing the usual formative power, but because there is 

 a physical impossibility of the two forces uniting, 

 through each of them pursuing an absolutely differ- 

 ent rhythm. 



