684 SEXUAL GENERATION. 



one animal are fecundated by the seminal fluid of another, and vice 

 versa. 



In all the vertebrate animals, on the other hand, the two sets of 

 generative organs are located in separate individuals ; and the species 

 is divided into two sexes, male and female. Beside this, there are, in 

 most instances, certain secondary or accessory organs of generation, 

 which assist in the accomplishment of the process, and which occasion 

 a corresponding difference in the anatomy of the two sexes. In some 

 cases this difference is so great that the male and female would never 

 be recognized as belonging to the same species, unless they were seen 

 in company with each other, and were known to reproduce the species 

 by sexual congress. Not to mention some extreme instances of this 

 among insects and other invertebrate animals, it is sufficient to refer to 

 the well-known examples of the cock and the hen, the lion and lioness, 

 the buck and the doe. In the human species, the distinction between 

 the sexes shows itself in the mental constitution, the disposition, habits, 

 and pursuits, as well as in the general conformation of the body, and 

 the external appearance. 



The special details of the generative process depend upon the struc- 

 ture of the male and female organs, the manner in which their products 

 are formed and discharged, the union of the two in the act of fecunda- 

 tion, and the changes which take place in the development of the 

 embryo. 



