WAYS OF ATTACKING THE PROBLEM 479 



Even in a type like-the rabbit it is possible, according to Russo, 

 to distinguish two kinds of ova in the ovary. In quite a number 



FIG. 45. Decorative male and less adorned female of Spathura a genus 

 of Humming-birds. (From Darwin, after Brehm.) 



of animals there is dimorphism of spermatozoa, though it is 

 not known what significance attaches to this. In some cases, 



