172 



EVOLUTION AXD ANIMAL LIFE 



stimulus. Among ants and the social bees and wasps the males 

 develop parthenogenetically from unfertilized cells, the fertilized 

 cells yielding either females or workers which are sterile females. 

 But this specialized mode of development is peculiar to particu- 

 lar groups. For a few lower species it has been ascertained 

 that variation in nutrition may be a factor in sex determination. 

 Favorable nutrition seems to increase the number of females. 

 Most higher plants are hermaphrodite, the central leaves (car- 

 pels) in the bud which becomes the flower, yielding ovules or fe- 



FiG. 104. — Limb skeletons of various animals, showing homologies of the bones; 

 at left, mole; next, giraffe; next, bat; next, porpoise. 



male germ cells. The next whorl (stamens) yields male germ 

 cells or pollen. The outer whorls (corolla, calyx) serve as pro- 

 tective organs only, and are without sex. 



The bonds of union among organisms which stand at the 

 basis of all classification are known as "homologies '^ (Figs. 101- 

 104). A homology is a real likeness, as distinguished from one, 

 merely superficial or apparent. To superficial likeness we give 

 the name of analogy. Homology means fundamental iden- 

 tity of structure, as distinguished from incidental similarit}^ of 

 form or function. Thus, the arm of a man is homologous with 

 the foreleg of a dog, because in either we can trace deep-seated 

 lesemblance or homologies with the other. In each detail of 

 each bone, nauscle, vein, or rierve of the one we can trace the 



