I 3 



ELEMENTARY ZOOLOGY. 



and their union gives marvelous powers to the new cell, 

 which neither had alone. In many egg cells there is the 

 secretion of some substance which serves to guide the 

 sperm toward the egg, through chemical sensitiveness. 

 But for this adaptation fertilization would be much less 

 sure. 



In the second place, the structure of the males and 

 females differs considerably and many of these structural 

 differences make it more easy to bring the sperm and the 

 ova together. 



Finally, there are many habits and instincts of both 

 males and females which help to insure mating and make 

 it effective. The color of the male bird, his motions, and 

 his peculiarly active song at mating time serve as recog- 

 nition marks whereby his presence is made known to the 

 female. In a similar way she has qualities that adjust 

 her to the male. These mutual adjustments extend even 

 to disposition. 



145. Practical Exercises. What is the difference in the notes of 

 the male and female of the American quail which would serve as 

 recognition marks? Mention other instances of differences between 

 the sexes, which appear to you to adjust one sex to the other in such 

 a way as to secure mating. . What value is there in the mingling of 

 sperm and ova? To what condition might the ability of ova to 

 develop without sperm (parthenogenesis) be an adaptation? 



146. Reproduction and Care of Young. One of the 



interesting respects in which animals vary is in the rate 

 of reproduction. Is there anything in the conditions of 

 life to which this rate is an adaptation? Why does one 

 animal produce only one young in a year and others 

 produce thousands? Of course the present-day individ- 

 ual does so because it is the nature of the species ; but 

 how did the species originally come to have this nature 



