THE REPRODUCTION OF ANIMALS 



225 



also reproduce asexually. There is good reason to believe that bisexual 

 reproduction confers special advantages on the organisms that practice 

 it. For one thing, it permits the development of a much more complex 

 degree of body organization than appears to be reproducible by asexual 

 means. The increased variability provided by biparental inheritance has 

 doubtless been an important factor in the evolution of the most successful 

 types of animal life. 



Whatever its advantages, it is also apparent that bisexual reproduction 

 encounters a complication not presented by asexual 6r unisexual methods. l 

 Fertilization of the egg requires the cooperation of two individuals, which 



Fig. 15.7. External fertilization with amplexus in the creek chub, Semotilus. The male 

 (above) and female (below) are seen over a pit scooped by the former in the gravel of a 

 stream riffle. As the eggs are laid and sink to the floor of the "nest" they are fertilized by 

 "milt" from the male, who later covers them with pebbles. (Redrawn, after Jacob Reighard.) 



not only must produce matured sperm and ova at the same season but 

 must, by appropriate behaviors, bring the spermatozoa and eggs into 

 close proximity. We find, therefore, that bisexual reproduction calls for 

 special breeding habits, which lead the males and females of the species 

 to abandon for a time their ordinary self-maintenance activities in order 

 to perform their roles as parents of the next generation. Here we often 

 find a clear subordination of the interests of the individual to the interests 

 of the race. Many of the habits that have been developed to accomplish 

 reproduction are highly dangerous or even fatal to the individuals that 

 perform them and result in a heavy mortality of parent organisms. 



Although the breeding habits of many species of higher metazoans 

 include parental care for the fertilized eggs and developing young, all 

 bisexual breeding habits begin with the special activities that lead to 

 the fertilization of the eggs. 



1 Except in the rare, and relatively unimportant, instances of self -fertilization in 

 hermaphroditic animals. 



