ORIGIN AND SIGNIFICANCE OF SEX. 231 



opment so long as the two inherited impulses were 

 alike, but when the impulses begin to impel growth 

 in opposite directions, the development must cease. 

 This explains why the imperfectly developed off- 

 spring of a crossed species resembles an ancestral 

 form. For, when the method of development of two 

 animals proceeds for a time on the same lines, and 

 then diverges, the result of the cross will develop 

 only as far as the point of divergence. Thus the 

 fancy breeds of pigeons, when crossed, produce the 

 ancestral form of the rock-pigeon. Also the stripes 

 on mules show a partial reversion to a zebra-like 

 ancestor of the horse and the ass. Since reproduc- 

 tion is a superabundant growth, these hybrid organ- 

 isms which do not attain a perfect development, nor 

 a full growth, are generally sterile. 



Conjugation, or union of two cells to produce the 

 one or more individuals of the succeeding genera- 

 tions, is generally admitted to be the first step in 

 the evolution of the sexual method of reproduction ; 

 and in this connection I wish to direct attention to 

 some points which have not generally received the 

 consideration which their importance seems to de- 

 mand. From its almost universal existence among 

 the more complex organisms, the sexual method of 

 reproduction, whereby each individual born may 

 inherit from two parental races, appears to play a 



